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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.23</releaseinfo>
10 <date>2014-01-06</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
43 <firstname>Sven</firstname>
44 <surname>Herzberg</surname>
46 <orgname>Imendio AB</orgname>
48 <email>sven@imendio.com</email>
53 <firstname>Simon</firstname>
54 <surname>McVittie</surname>
56 <orgname>Collabora Ltd.</orgname>
58 <email>simon.mcvittie@collabora.co.uk</email>
63 <firstname>David</firstname>
64 <surname>Zeuthen</surname>
67 <email>zeuthen@gmail.com</email>
74 <revnumber>0.23</revnumber>
75 <date>2014-01-06</date>
76 <authorinitials>SMcV, CY</authorinitials>
78 method call messages with no INTERFACE may be considered an error;
79 document tcp:bind=... and nonce-tcp:bind=...; define listenable
80 and connectable addresses
84 <revnumber>0.22</revnumber>
85 <date>2013-10-09</date>
86 <authorinitials></authorinitials>
87 <revremark>add GetConnectionCredentials, document
88 GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
89 document and correct .service file syntax and naming
93 <revnumber>0.21</revnumber>
94 <date>2013-04-25</date>
95 <authorinitials>smcv</authorinitials>
96 <revremark>allow Unicode noncharacters in UTF-8 (Unicode
97 Corrigendum #9)</revremark>
100 <revnumber>0.20</revnumber>
101 <date>22 February 2013</date>
102 <authorinitials>smcv, walters</authorinitials>
103 <revremark>reorganise for clarity, remove false claims about
104 basic types, mention /o/fd/DBus</revremark>
107 <revnumber>0.19</revnumber>
108 <date>20 February 2012</date>
109 <authorinitials>smcv/lp</authorinitials>
110 <revremark>formally define unique connection names and well-known
111 bus names; document best practices for interface, bus, member and
112 error names, and object paths; document the search path for session
113 and system services on Unix; document the systemd transport</revremark>
116 <revnumber>0.18</revnumber>
117 <date>29 July 2011</date>
118 <authorinitials>smcv</authorinitials>
119 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
120 match keyword; promote type system to a top-level section</revremark>
123 <revnumber>0.17</revnumber>
124 <date>1 June 2011</date>
125 <authorinitials>smcv/davidz</authorinitials>
126 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
127 by GVariant</revremark>
130 <revnumber>0.16</revnumber>
131 <date>11 April 2011</date>
132 <authorinitials></authorinitials>
133 <revremark>add path_namespace, arg0namespace; argNpath matches object
137 <revnumber>0.15</revnumber>
138 <date>3 November 2010</date>
139 <authorinitials></authorinitials>
140 <revremark></revremark>
143 <revnumber>0.14</revnumber>
144 <date>12 May 2010</date>
145 <authorinitials></authorinitials>
146 <revremark></revremark>
149 <revnumber>0.13</revnumber>
150 <date>23 Dezember 2009</date>
151 <authorinitials></authorinitials>
152 <revremark></revremark>
155 <revnumber>0.12</revnumber>
156 <date>7 November, 2006</date>
157 <authorinitials></authorinitials>
158 <revremark></revremark>
161 <revnumber>0.11</revnumber>
162 <date>6 February 2005</date>
163 <authorinitials></authorinitials>
164 <revremark></revremark>
167 <revnumber>0.10</revnumber>
168 <date>28 January 2005</date>
169 <authorinitials></authorinitials>
170 <revremark></revremark>
173 <revnumber>0.9</revnumber>
174 <date>7 Januar 2005</date>
175 <authorinitials></authorinitials>
176 <revremark></revremark>
179 <revnumber>0.8</revnumber>
180 <date>06 September 2003</date>
181 <authorinitials></authorinitials>
182 <revremark>First released document.</revremark>
187 <sect1 id="introduction">
188 <title>Introduction</title>
190 D-Bus is a system for low-overhead, easy to use
191 interprocess communication (IPC). In more detail:
195 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
196 binary protocol, and does not have to convert to and from a text
197 format such as XML. Because D-Bus is intended for potentially
198 high-resolution same-machine IPC, not primarily for Internet IPC,
199 this is an interesting optimization. D-Bus is also designed to
200 avoid round trips and allow asynchronous operation, much like
206 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
207 of <firstterm>messages</firstterm> rather than byte streams, and
208 automatically handles a lot of the hard IPC issues. Also, the D-Bus
209 library is designed to be wrapped in a way that lets developers use
210 their framework's existing object/type system, rather than learning
211 a new one specifically for IPC.
218 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
219 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
220 a system for one application to talk to a single other
221 application. However, the primary intended application of the protocol is the
222 D-Bus <firstterm>message bus</firstterm>, specified in <xref
223 linkend="message-bus"/>. The message bus is a special application that
224 accepts connections from multiple other applications, and forwards
229 Uses of D-Bus include notification of system changes (notification of when
230 a camera is plugged in to a computer, or a new version of some software
231 has been installed), or desktop interoperability, for example a file
232 monitoring service or a configuration service.
236 D-Bus is designed for two specific use cases:
240 A "system bus" for notifications from the system to user sessions,
241 and to allow the system to request input from user sessions.
246 A "session bus" used to implement desktop environments such as
251 D-Bus is not intended to be a generic IPC system for any possible
252 application, and intentionally omits many features found in other
253 IPC systems for this reason.
257 At the same time, the bus daemons offer a number of features not found in
258 other IPC systems, such as single-owner "bus names" (similar to X
259 selections), on-demand startup of services, and security policies.
260 In many ways, these features are the primary motivation for developing
261 D-Bus; other systems would have sufficed if IPC were the only goal.
265 D-Bus may turn out to be useful in unanticipated applications, but future
266 versions of this spec and the reference implementation probably will not
267 incorporate features that interfere with the core use cases.
271 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
272 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
273 document are to be interpreted as described in RFC 2119. However, the
274 document could use a serious audit to be sure it makes sense to do
275 so. Also, they are not capitalized.
278 <sect2 id="stability">
279 <title>Protocol and Specification Stability</title>
281 The D-Bus protocol is frozen (only compatible extensions are allowed) as
282 of November 8, 2006. However, this specification could still use a fair
283 bit of work to make interoperable reimplementation possible without
284 reference to the D-Bus reference implementation. Thus, this
285 specification is not marked 1.0. To mark it 1.0, we'd like to see
286 someone invest significant effort in clarifying the specification
287 language, and growing the specification to cover more aspects of the
288 reference implementation's behavior.
291 Until this work is complete, any attempt to reimplement D-Bus will
292 probably require looking at the reference implementation and/or asking
293 questions on the D-Bus mailing list about intended behavior.
294 Questions on the list are very welcome.
297 Nonetheless, this document should be a useful starting point and is
298 to our knowledge accurate, though incomplete.
304 <sect1 id="type-system">
305 <title>Type System</title>
308 D-Bus has a type system, in which values of various types can be
309 serialized into a sequence of bytes referred to as the
310 <firstterm>wire format</firstterm> in a standard way.
311 Converting a value from some other representation into the wire
312 format is called <firstterm>marshaling</firstterm> and converting
313 it back from the wire format is <firstterm>unmarshaling</firstterm>.
317 The D-Bus protocol does not include type tags in the marshaled data; a
318 block of marshaled values must have a known <firstterm>type
319 signature</firstterm>. The type signature is made up of zero or more
320 <firstterm id="term-single-complete-type">single complete
321 types</firstterm>, each made up of one or more
322 <firstterm>type codes</firstterm>.
326 A type code is an ASCII character representing the
327 type of a value. Because ASCII characters are used, the type signature
328 will always form a valid ASCII string. A simple string compare
329 determines whether two type signatures are equivalent.
333 A single complete type is a sequence of type codes that fully describes
334 one type: either a basic type, or a single fully-described container type.
335 A single complete type is a basic type code, a variant type code,
336 an array with its element type, or a struct with its fields (all of which
337 are defined below). So the following signatures are not single complete
348 And the following signatures contain multiple complete types:
358 Note however that a single complete type may <emphasis>contain</emphasis>
359 multiple other single complete types, by containing a struct or dict
363 <sect2 id="basic-types">
364 <title>Basic types</title>
367 The simplest type codes are the <firstterm id="term-basic-type">basic
368 types</firstterm>, which are the types whose structure is entirely
369 defined by their 1-character type code. Basic types consist of
370 fixed types and string-like types.
374 The <firstterm id="term-fixed-type">fixed types</firstterm>
375 are basic types whose values have a fixed length, namely BYTE,
376 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
381 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
382 the ASCII character 'i'. So the signature for a block of values
383 containing a single <literal>INT32</literal> would be:
387 A block of values containing two <literal>INT32</literal> would have this signature:
394 The characteristics of the fixed types are listed in this table.
400 <entry>Conventional name</entry>
401 <entry>ASCII type-code</entry>
402 <entry>Encoding</entry>
407 <entry><literal>BYTE</literal></entry>
408 <entry><literal>y</literal> (121)</entry>
409 <entry>Unsigned 8-bit integer</entry>
412 <entry><literal>BOOLEAN</literal></entry>
413 <entry><literal>b</literal> (98)</entry>
414 <entry>Boolean value: 0 is false, 1 is true, any other value
415 allowed by the marshalling format is invalid</entry>
418 <entry><literal>INT16</literal></entry>
419 <entry><literal>n</literal> (110)</entry>
420 <entry>Signed (two's complement) 16-bit integer</entry>
423 <entry><literal>UINT16</literal></entry>
424 <entry><literal>q</literal> (113)</entry>
425 <entry>Unsigned 16-bit integer</entry>
428 <entry><literal>INT32</literal></entry>
429 <entry><literal>i</literal> (105)</entry>
430 <entry>Signed (two's complement) 32-bit integer</entry>
433 <entry><literal>UINT32</literal></entry>
434 <entry><literal>u</literal> (117)</entry>
435 <entry>Unsigned 32-bit integer</entry>
438 <entry><literal>INT64</literal></entry>
439 <entry><literal>x</literal> (120)</entry>
440 <entry>Signed (two's complement) 64-bit integer
441 (mnemonic: x and t are the first characters in "sixty" not
442 already used for something more common)</entry>
445 <entry><literal>UINT64</literal></entry>
446 <entry><literal>t</literal> (116)</entry>
447 <entry>Unsigned 64-bit integer</entry>
450 <entry><literal>DOUBLE</literal></entry>
451 <entry><literal>d</literal> (100)</entry>
452 <entry>IEEE 754 double-precision floating point</entry>
455 <entry><literal>UNIX_FD</literal></entry>
456 <entry><literal>h</literal> (104)</entry>
457 <entry>Unsigned 32-bit integer representing an index into an
458 out-of-band array of file descriptors, transferred via some
459 platform-specific mechanism (mnemonic: h for handle)</entry>
467 The <firstterm id="term-string-like-type">string-like types</firstterm>
468 are basic types with a variable length. The value of any string-like
469 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
470 none of which may be U+0000. The UTF-8 text must be validated
471 strictly: in particular, it must not contain overlong sequences
472 or codepoints above U+10FFFF.
476 Since D-Bus Specification version 0.21, in accordance with Unicode
477 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
478 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
479 D-Bus rejected these noncharacters).
483 The marshalling formats for the string-like types all end with a
484 single zero (NUL) byte, but that byte is not considered to be part of
489 The characteristics of the string-like types are listed in this table.
495 <entry>Conventional name</entry>
496 <entry>ASCII type-code</entry>
497 <entry>Validity constraints</entry>
502 <entry><literal>STRING</literal></entry>
503 <entry><literal>s</literal> (115)</entry>
504 <entry>No extra constraints</entry>
507 <entry><literal>OBJECT_PATH</literal></entry>
508 <entry><literal>o</literal> (111)</entry>
510 <link linkend="message-protocol-marshaling-object-path">a
511 syntactically valid object path</link></entry>
514 <entry><literal>SIGNATURE</literal></entry>
515 <entry><literal>g</literal> (103)</entry>
517 <firstterm linkend="term-single-complete-type">single
518 complete types</firstterm></entry>
525 <sect3 id="message-protocol-marshaling-object-path">
526 <title>Valid Object Paths</title>
529 An object path is a name used to refer to an object instance.
530 Conceptually, each participant in a D-Bus message exchange may have
531 any number of object instances (think of C++ or Java objects) and each
532 such instance will have a path. Like a filesystem, the object
533 instances in an application form a hierarchical tree.
537 Object paths are often namespaced by starting with a reversed
538 domain name and containing an interface version number, in the
540 <link linkend="message-protocol-names-interface">interface
542 <link linkend="message-protocol-names-bus">well-known
544 This makes it possible to implement more than one service, or
545 more than one version of a service, in the same process,
546 even if the services share a connection but cannot otherwise
547 co-operate (for instance, if they are implemented by different
552 For instance, if the owner of <literal>example.com</literal> is
553 developing a D-Bus API for a music player, they might use the
554 hierarchy of object paths that start with
555 <literal>/com/example/MusicPlayer1</literal> for its objects.
559 The following rules define a valid object path. Implementations must
560 not send or accept messages with invalid object paths.
564 The path may be of any length.
569 The path must begin with an ASCII '/' (integer 47) character,
570 and must consist of elements separated by slash characters.
575 Each element must only contain the ASCII characters
581 No element may be the empty string.
586 Multiple '/' characters cannot occur in sequence.
591 A trailing '/' character is not allowed unless the
592 path is the root path (a single '/' character).
600 <sect3 id="message-protocol-marshaling-signature">
601 <title>Valid Signatures</title>
603 An implementation must not send or accept invalid signatures.
604 Valid signatures will conform to the following rules:
608 The signature is a list of single complete types.
609 Arrays must have element types, and structs must
610 have both open and close parentheses.
615 Only type codes, open and close parentheses, and open and
616 close curly brackets are allowed in the signature. The
617 <literal>STRUCT</literal> type code
618 is not allowed in signatures, because parentheses
619 are used instead. Similarly, the
620 <literal>DICT_ENTRY</literal> type code is not allowed in
621 signatures, because curly brackets are used instead.
626 The maximum depth of container type nesting is 32 array type
627 codes and 32 open parentheses. This implies that the maximum
628 total depth of recursion is 64, for an "array of array of array
629 of ... struct of struct of struct of ..." where there are 32
635 The maximum length of a signature is 255.
642 When signatures appear in messages, the marshalling format
643 guarantees that they will be followed by a nul byte (which can
644 be interpreted as either C-style string termination or the INVALID
645 type-code), but this is not conceptually part of the signature.
651 <sect2 id="container-types">
652 <title>Container types</title>
655 In addition to basic types, there are four <firstterm>container</firstterm>
656 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
657 and <literal>DICT_ENTRY</literal>.
661 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
662 code does not appear in signatures. Instead, ASCII characters
663 '(' and ')' are used to mark the beginning and end of the struct.
664 So for example, a struct containing two integers would have this
669 Structs can be nested, so for example a struct containing
670 an integer and another struct:
674 The value block storing that struct would contain three integers; the
675 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
680 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
681 but is useful in code that implements the protocol. This type code
682 is specified to allow such code to interoperate in non-protocol contexts.
686 Empty structures are not allowed; there must be at least one
687 type code between the parentheses.
691 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
692 followed by a <firstterm>single complete type</firstterm>. The single
693 complete type following the array is the type of each array element. So
694 the simple example is:
698 which is an array of 32-bit integers. But an array can be of any type,
699 such as this array-of-struct-with-two-int32-fields:
703 Or this array of array of integer:
710 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
711 type <literal>VARIANT</literal> will have the signature of a single complete type as part
712 of the <emphasis>value</emphasis>. This signature will be followed by a
713 marshaled value of that type.
717 Unlike a message signature, the variant signature can
718 contain only a single complete type. So "i", "ai"
719 or "(ii)" is OK, but "ii" is not. Use of variants may not
720 cause a total message depth to be larger than 64, including
721 other container types such as structures.
725 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
726 than parentheses it uses curly braces, and it has more restrictions.
727 The restrictions are: it occurs only as an array element type; it has
728 exactly two single complete types inside the curly braces; the first
729 single complete type (the "key") must be a basic type rather than a
730 container type. Implementations must not accept dict entries outside of
731 arrays, must not accept dict entries with zero, one, or more than two
732 fields, and must not accept dict entries with non-basic-typed keys. A
733 dict entry is always a key-value pair.
737 The first field in the <literal>DICT_ENTRY</literal> is always the key.
738 A message is considered corrupt if the same key occurs twice in the same
739 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
740 implementations are not required to reject dicts with duplicate keys.
744 In most languages, an array of dict entry would be represented as a
745 map, hash table, or dict object.
750 <title>Summary of types</title>
753 The following table summarizes the D-Bus types.
758 <entry>Conventional Name</entry>
760 <entry>Description</entry>
765 <entry><literal>INVALID</literal></entry>
766 <entry>0 (ASCII NUL)</entry>
767 <entry>Not a valid type code, used to terminate signatures</entry>
769 <entry><literal>BYTE</literal></entry>
770 <entry>121 (ASCII 'y')</entry>
771 <entry>8-bit unsigned integer</entry>
773 <entry><literal>BOOLEAN</literal></entry>
774 <entry>98 (ASCII 'b')</entry>
775 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
777 <entry><literal>INT16</literal></entry>
778 <entry>110 (ASCII 'n')</entry>
779 <entry>16-bit signed integer</entry>
781 <entry><literal>UINT16</literal></entry>
782 <entry>113 (ASCII 'q')</entry>
783 <entry>16-bit unsigned integer</entry>
785 <entry><literal>INT32</literal></entry>
786 <entry>105 (ASCII 'i')</entry>
787 <entry>32-bit signed integer</entry>
789 <entry><literal>UINT32</literal></entry>
790 <entry>117 (ASCII 'u')</entry>
791 <entry>32-bit unsigned integer</entry>
793 <entry><literal>INT64</literal></entry>
794 <entry>120 (ASCII 'x')</entry>
795 <entry>64-bit signed integer</entry>
797 <entry><literal>UINT64</literal></entry>
798 <entry>116 (ASCII 't')</entry>
799 <entry>64-bit unsigned integer</entry>
801 <entry><literal>DOUBLE</literal></entry>
802 <entry>100 (ASCII 'd')</entry>
803 <entry>IEEE 754 double</entry>
805 <entry><literal>STRING</literal></entry>
806 <entry>115 (ASCII 's')</entry>
807 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
809 <entry><literal>OBJECT_PATH</literal></entry>
810 <entry>111 (ASCII 'o')</entry>
811 <entry>Name of an object instance</entry>
813 <entry><literal>SIGNATURE</literal></entry>
814 <entry>103 (ASCII 'g')</entry>
815 <entry>A type signature</entry>
817 <entry><literal>ARRAY</literal></entry>
818 <entry>97 (ASCII 'a')</entry>
821 <entry><literal>STRUCT</literal></entry>
822 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
823 <entry>Struct; type code 114 'r' is reserved for use in
824 bindings and implementations to represent the general
825 concept of a struct, and must not appear in signatures
826 used on D-Bus.</entry>
828 <entry><literal>VARIANT</literal></entry>
829 <entry>118 (ASCII 'v') </entry>
830 <entry>Variant type (the type of the value is part of the value itself)</entry>
832 <entry><literal>DICT_ENTRY</literal></entry>
833 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
834 <entry>Entry in a dict or map (array of key-value pairs).
835 Type code 101 'e' is reserved for use in bindings and
836 implementations to represent the general concept of a
837 dict or dict-entry, and must not appear in signatures
838 used on D-Bus.</entry>
840 <entry><literal>UNIX_FD</literal></entry>
841 <entry>104 (ASCII 'h')</entry>
842 <entry>Unix file descriptor</entry>
845 <entry>(reserved)</entry>
846 <entry>109 (ASCII 'm')</entry>
847 <entry>Reserved for <ulink
848 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
849 'maybe' type compatible with the one in GVariant</ulink>,
850 and must not appear in signatures used on D-Bus until
851 specified here</entry>
854 <entry>(reserved)</entry>
855 <entry>42 (ASCII '*')</entry>
856 <entry>Reserved for use in bindings/implementations to
857 represent any <firstterm>single complete type</firstterm>,
858 and must not appear in signatures used on D-Bus.</entry>
861 <entry>(reserved)</entry>
862 <entry>63 (ASCII '?')</entry>
863 <entry>Reserved for use in bindings/implementations to
864 represent any <firstterm>basic type</firstterm>, and must
865 not appear in signatures used on D-Bus.</entry>
868 <entry>(reserved)</entry>
869 <entry>64 (ASCII '@'), 38 (ASCII '&'),
870 94 (ASCII '^')</entry>
871 <entry>Reserved for internal use by bindings/implementations,
872 and must not appear in signatures used on D-Bus.
873 GVariant uses these type-codes to encode calling
884 <sect1 id="message-protocol-marshaling">
885 <title>Marshaling (Wire Format)</title>
888 D-Bus defines a marshalling format for its type system, which is
889 used in D-Bus messages. This is not the only possible marshalling
890 format for the type system: for instance, GVariant (part of GLib)
891 re-uses the D-Bus type system but implements an alternative marshalling
896 <title>Byte order and alignment</title>
899 Given a type signature, a block of bytes can be converted into typed
900 values. This section describes the format of the block of bytes. Byte
901 order and alignment issues are handled uniformly for all D-Bus types.
905 A block of bytes has an associated byte order. The byte order
906 has to be discovered in some way; for D-Bus messages, the
907 byte order is part of the message header as described in
908 <xref linkend="message-protocol-messages"/>. For now, assume
909 that the byte order is known to be either little endian or big
914 Each value in a block of bytes is aligned "naturally," for example
915 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
916 8-byte boundary. To properly align a value, <firstterm>alignment
917 padding</firstterm> may be necessary. The alignment padding must always
918 be the minimum required padding to properly align the following value;
919 and it must always be made up of nul bytes. The alignment padding must
920 not be left uninitialized (it can't contain garbage), and more padding
921 than required must not be used.
925 As an exception to natural alignment, <literal>STRUCT</literal> and
926 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
927 boundary, regardless of the alignments of their contents.
932 <title>Marshalling basic types</title>
935 To marshal and unmarshal fixed types, you simply read one value
936 from the data block corresponding to each type code in the signature.
937 All signed integer values are encoded in two's complement, DOUBLE
938 values are IEEE 754 double-precision floating-point, and BOOLEAN
939 values are encoded in 32 bits (of which only the least significant
944 The string-like types are all marshalled as a
945 fixed-length unsigned integer <varname>n</varname> giving the
946 length of the variable part, followed by <varname>n</varname>
947 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
948 which is not considered to be part of the text. The alignment
949 of the string-like type is the same as the alignment of
950 <varname>n</varname>.
954 For the STRING and OBJECT_PATH types, <varname>n</varname> is
955 encoded in 4 bytes, leading to 4-byte alignment.
956 For the SIGNATURE type, <varname>n</varname> is encoded as a single
957 byte. As a result, alignment padding is never required before a
963 <title>Marshalling containers</title>
966 Arrays are marshalled as a <literal>UINT32</literal>
967 <varname>n</varname> giving the length of the array data in bytes,
968 followed by alignment padding to the alignment boundary of the array
969 element type, followed by the <varname>n</varname> bytes of the
970 array elements marshalled in sequence. <varname>n</varname> does not
971 include the padding after the length, or any padding after the
976 For instance, if the current position in the message is a multiple
977 of 8 bytes and the byte-order is big-endian, an array containing only
978 the 64-bit integer 5 would be marshalled as:
981 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
982 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
983 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
988 Arrays have a maximum length defined to be 2 to the 26th power or
989 67108864. Implementations must not send or accept arrays exceeding this
994 Structs and dict entries are marshalled in the same way as their
995 contents, but their alignment is always to an 8-byte boundary,
996 even if their contents would normally be less strictly aligned.
1000 Variants are marshalled as the <literal>SIGNATURE</literal> of
1001 the contents (which must be a single complete type), followed by a
1002 marshalled value with the type given by that signature. The
1003 variant has the same 1-byte alignment as the signature, which means
1004 that alignment padding before a variant is never needed.
1005 Use of variants may not cause a total message depth to be larger
1006 than 64, including other container types such as structures.
1011 <title>Summary of D-Bus marshalling</title>
1014 Given all this, the types are marshaled on the wire as follows:
1019 <entry>Conventional Name</entry>
1020 <entry>Encoding</entry>
1021 <entry>Alignment</entry>
1026 <entry><literal>INVALID</literal></entry>
1027 <entry>Not applicable; cannot be marshaled.</entry>
1030 <entry><literal>BYTE</literal></entry>
1031 <entry>A single 8-bit byte.</entry>
1034 <entry><literal>BOOLEAN</literal></entry>
1035 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1038 <entry><literal>INT16</literal></entry>
1039 <entry>16-bit signed integer in the message's byte order.</entry>
1042 <entry><literal>UINT16</literal></entry>
1043 <entry>16-bit unsigned integer in the message's byte order.</entry>
1046 <entry><literal>INT32</literal></entry>
1047 <entry>32-bit signed integer in the message's byte order.</entry>
1050 <entry><literal>UINT32</literal></entry>
1051 <entry>32-bit unsigned integer in the message's byte order.</entry>
1054 <entry><literal>INT64</literal></entry>
1055 <entry>64-bit signed integer in the message's byte order.</entry>
1058 <entry><literal>UINT64</literal></entry>
1059 <entry>64-bit unsigned integer in the message's byte order.</entry>
1062 <entry><literal>DOUBLE</literal></entry>
1063 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1066 <entry><literal>STRING</literal></entry>
1067 <entry>A <literal>UINT32</literal> indicating the string's
1068 length in bytes excluding its terminating nul, followed by
1069 non-nul string data of the given length, followed by a terminating nul
1076 <entry><literal>OBJECT_PATH</literal></entry>
1077 <entry>Exactly the same as <literal>STRING</literal> except the
1078 content must be a valid object path (see above).
1084 <entry><literal>SIGNATURE</literal></entry>
1085 <entry>The same as <literal>STRING</literal> except the length is a single
1086 byte (thus signatures have a maximum length of 255)
1087 and the content must be a valid signature (see above).
1093 <entry><literal>ARRAY</literal></entry>
1095 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1096 alignment padding to the alignment boundary of the array element type,
1097 followed by each array element.
1103 <entry><literal>STRUCT</literal></entry>
1105 A struct must start on an 8-byte boundary regardless of the
1106 type of the struct fields. The struct value consists of each
1107 field marshaled in sequence starting from that 8-byte
1114 <entry><literal>VARIANT</literal></entry>
1116 The marshaled <literal>SIGNATURE</literal> of a single
1117 complete type, followed by a marshaled value with the type
1118 given in the signature.
1121 1 (alignment of the signature)
1124 <entry><literal>DICT_ENTRY</literal></entry>
1126 Identical to STRUCT.
1132 <entry><literal>UNIX_FD</literal></entry>
1133 <entry>32-bit unsigned integer in the message's byte
1134 order. The actual file descriptors need to be
1135 transferred out-of-band via some platform specific
1136 mechanism. On the wire, values of this type store the index to the
1137 file descriptor in the array of file descriptors that
1138 accompany the message.</entry>
1150 <sect1 id="message-protocol">
1151 <title>Message Protocol</title>
1154 A <firstterm>message</firstterm> consists of a
1155 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1156 think of a message as a package, the header is the address, and the body
1157 contains the package contents. The message delivery system uses the header
1158 information to figure out where to send the message and how to interpret
1159 it; the recipient interprets the body of the message.
1163 The body of the message is made up of zero or more
1164 <firstterm>arguments</firstterm>, which are typed values, such as an
1165 integer or a byte array.
1169 Both header and body use the D-Bus <link linkend="type-system">type
1170 system</link> and format for serializing data.
1173 <sect2 id="message-protocol-messages">
1174 <title>Message Format</title>
1177 A message consists of a header and a body. The header is a block of
1178 values with a fixed signature and meaning. The body is a separate block
1179 of values, with a signature specified in the header.
1183 The length of the header must be a multiple of 8, allowing the body to
1184 begin on an 8-byte boundary when storing the entire message in a single
1185 buffer. If the header does not naturally end on an 8-byte boundary
1186 up to 7 bytes of nul-initialized alignment padding must be added.
1190 The message body need not end on an 8-byte boundary.
1194 The maximum length of a message, including header, header alignment padding,
1195 and body is 2 to the 27th power or 134217728. Implementations must not
1196 send or accept messages exceeding this size.
1200 The signature of the header is:
1204 Written out more readably, this is:
1206 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1211 These values have the following meanings:
1216 <entry>Value</entry>
1217 <entry>Description</entry>
1222 <entry>1st <literal>BYTE</literal></entry>
1223 <entry>Endianness flag; ASCII 'l' for little-endian
1224 or ASCII 'B' for big-endian. Both header and body are
1225 in this endianness.</entry>
1228 <entry>2nd <literal>BYTE</literal></entry>
1229 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1230 Currently-defined types are described below.
1234 <entry>3rd <literal>BYTE</literal></entry>
1235 <entry>Bitwise OR of flags. Unknown flags
1236 must be ignored. Currently-defined flags are described below.
1240 <entry>4th <literal>BYTE</literal></entry>
1241 <entry>Major protocol version of the sending application. If
1242 the major protocol version of the receiving application does not
1243 match, the applications will not be able to communicate and the
1244 D-Bus connection must be disconnected. The major protocol
1245 version for this version of the specification is 1.
1249 <entry>1st <literal>UINT32</literal></entry>
1250 <entry>Length in bytes of the message body, starting
1251 from the end of the header. The header ends after
1252 its alignment padding to an 8-boundary.
1256 <entry>2nd <literal>UINT32</literal></entry>
1257 <entry>The serial of this message, used as a cookie
1258 by the sender to identify the reply corresponding
1259 to this request. This must not be zero.
1263 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1264 <entry>An array of zero or more <firstterm>header
1265 fields</firstterm> where the byte is the field code, and the
1266 variant is the field value. The message type determines
1267 which fields are required.
1275 <firstterm>Message types</firstterm> that can appear in the second byte
1281 <entry>Conventional name</entry>
1282 <entry>Decimal value</entry>
1283 <entry>Description</entry>
1288 <entry><literal>INVALID</literal></entry>
1290 <entry>This is an invalid type.</entry>
1293 <entry><literal>METHOD_CALL</literal></entry>
1295 <entry>Method call.</entry>
1298 <entry><literal>METHOD_RETURN</literal></entry>
1300 <entry>Method reply with returned data.</entry>
1303 <entry><literal>ERROR</literal></entry>
1305 <entry>Error reply. If the first argument exists and is a
1306 string, it is an error message.</entry>
1309 <entry><literal>SIGNAL</literal></entry>
1311 <entry>Signal emission.</entry>
1318 Flags that can appear in the third byte of the header:
1323 <entry>Conventional name</entry>
1324 <entry>Hex value</entry>
1325 <entry>Description</entry>
1330 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1332 <entry>This message does not expect method return replies or
1333 error replies; the reply can be omitted as an
1334 optimization. However, it is compliant with this specification
1335 to return the reply despite this flag and the only harm
1336 from doing so is extra network traffic.
1340 <entry><literal>NO_AUTO_START</literal></entry>
1342 <entry>The bus must not launch an owner
1343 for the destination name in response to this message.
1351 <sect3 id="message-protocol-header-fields">
1352 <title>Header Fields</title>
1355 The array at the end of the header contains <firstterm>header
1356 fields</firstterm>, where each field is a 1-byte field code followed
1357 by a field value. A header must contain the required header fields for
1358 its message type, and zero or more of any optional header
1359 fields. Future versions of this protocol specification may add new
1360 fields. Implementations must ignore fields they do not
1361 understand. Implementations must not invent their own header fields;
1362 only changes to this specification may introduce new header fields.
1366 Again, if an implementation sees a header field code that it does not
1367 expect, it must ignore that field, as it will be part of a new
1368 (but compatible) version of this specification. This also applies
1369 to known header fields appearing in unexpected messages, for
1370 example: if a signal has a reply serial it must be ignored
1371 even though it has no meaning as of this version of the spec.
1375 However, implementations must not send or accept known header fields
1376 with the wrong type stored in the field value. So for example a
1377 message with an <literal>INTERFACE</literal> field of type
1378 <literal>UINT32</literal> would be considered corrupt.
1382 Here are the currently-defined header fields:
1387 <entry>Conventional Name</entry>
1388 <entry>Decimal Code</entry>
1390 <entry>Required In</entry>
1391 <entry>Description</entry>
1396 <entry><literal>INVALID</literal></entry>
1399 <entry>not allowed</entry>
1400 <entry>Not a valid field name (error if it appears in a message)</entry>
1403 <entry><literal>PATH</literal></entry>
1405 <entry><literal>OBJECT_PATH</literal></entry>
1406 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1407 <entry>The object to send a call to,
1408 or the object a signal is emitted from.
1410 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1411 implementations should not send messages with this path,
1412 and the reference implementation of the bus daemon will
1413 disconnect any application that attempts to do so.
1417 <entry><literal>INTERFACE</literal></entry>
1419 <entry><literal>STRING</literal></entry>
1420 <entry><literal>SIGNAL</literal></entry>
1422 The interface to invoke a method call on, or
1423 that a signal is emitted from. Optional for
1424 method calls, required for signals.
1425 The special interface
1426 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1427 implementations should not send messages with this
1428 interface, and the reference implementation of the bus
1429 daemon will disconnect any application that attempts to
1434 <entry><literal>MEMBER</literal></entry>
1436 <entry><literal>STRING</literal></entry>
1437 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1438 <entry>The member, either the method name or signal name.</entry>
1441 <entry><literal>ERROR_NAME</literal></entry>
1443 <entry><literal>STRING</literal></entry>
1444 <entry><literal>ERROR</literal></entry>
1445 <entry>The name of the error that occurred, for errors</entry>
1448 <entry><literal>REPLY_SERIAL</literal></entry>
1450 <entry><literal>UINT32</literal></entry>
1451 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1452 <entry>The serial number of the message this message is a reply
1453 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1456 <entry><literal>DESTINATION</literal></entry>
1458 <entry><literal>STRING</literal></entry>
1459 <entry>optional</entry>
1460 <entry>The name of the connection this message is intended for.
1461 Only used in combination with the message bus, see
1462 <xref linkend="message-bus"/>.</entry>
1465 <entry><literal>SENDER</literal></entry>
1467 <entry><literal>STRING</literal></entry>
1468 <entry>optional</entry>
1469 <entry>Unique name of the sending connection.
1470 The message bus fills in this field so it is reliable; the field is
1471 only meaningful in combination with the message bus.</entry>
1474 <entry><literal>SIGNATURE</literal></entry>
1476 <entry><literal>SIGNATURE</literal></entry>
1477 <entry>optional</entry>
1478 <entry>The signature of the message body.
1479 If omitted, it is assumed to be the
1480 empty signature "" (i.e. the body must be 0-length).</entry>
1483 <entry><literal>UNIX_FDS</literal></entry>
1485 <entry><literal>UINT32</literal></entry>
1486 <entry>optional</entry>
1487 <entry>The number of Unix file descriptors that
1488 accompany the message. If omitted, it is assumed
1489 that no Unix file descriptors accompany the
1490 message. The actual file descriptors need to be
1491 transferred via platform specific mechanism
1492 out-of-band. They must be sent at the same time as
1493 part of the message itself. They may not be sent
1494 before the first byte of the message itself is
1495 transferred or after the last byte of the message
1505 <sect2 id="message-protocol-names">
1506 <title>Valid Names</title>
1508 The various names in D-Bus messages have some restrictions.
1511 There is a <firstterm>maximum name length</firstterm>
1512 of 255 which applies to bus names, interfaces, and members.
1514 <sect3 id="message-protocol-names-interface">
1515 <title>Interface names</title>
1517 Interfaces have names with type <literal>STRING</literal>, meaning that
1518 they must be valid UTF-8. However, there are also some
1519 additional restrictions that apply to interface names
1522 <listitem><para>Interface names are composed of 1 or more elements separated by
1523 a period ('.') character. All elements must contain at least
1527 <listitem><para>Each element must only contain the ASCII characters
1528 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1532 <listitem><para>Interface names must contain at least one '.' (period)
1533 character (and thus at least two elements).
1536 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1537 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1542 Interface names should start with the reversed DNS domain name of
1543 the author of the interface (in lower-case), like interface names
1544 in Java. It is conventional for the rest of the interface name
1545 to consist of words run together, with initial capital letters
1546 on all words ("CamelCase"). Several levels of hierarchy can be used.
1547 It is also a good idea to include the major version of the interface
1548 in the name, and increment it if incompatible changes are made;
1549 this way, a single object can implement several versions of an
1550 interface in parallel, if necessary.
1554 For instance, if the owner of <literal>example.com</literal> is
1555 developing a D-Bus API for a music player, they might define
1556 interfaces called <literal>com.example.MusicPlayer1</literal>,
1557 <literal>com.example.MusicPlayer1.Track</literal> and
1558 <literal>com.example.MusicPlayer1.Seekable</literal>.
1562 D-Bus does not distinguish between the concepts that would be
1563 called classes and interfaces in Java: either can be identified on
1564 D-Bus by an interface name.
1567 <sect3 id="message-protocol-names-bus">
1568 <title>Bus names</title>
1570 Connections have one or more bus names associated with them.
1571 A connection has exactly one bus name that is a <firstterm>unique
1572 connection name</firstterm>. The unique connection name remains
1573 with the connection for its entire lifetime.
1574 A bus name is of type <literal>STRING</literal>,
1575 meaning that it must be valid UTF-8. However, there are also
1576 some additional restrictions that apply to bus names
1579 <listitem><para>Bus names that start with a colon (':')
1580 character are unique connection names. Other bus names
1581 are called <firstterm>well-known bus names</firstterm>.
1584 <listitem><para>Bus names are composed of 1 or more elements separated by
1585 a period ('.') character. All elements must contain at least
1589 <listitem><para>Each element must only contain the ASCII characters
1590 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1591 connection name may begin with a digit, elements in
1592 other bus names must not begin with a digit.
1596 <listitem><para>Bus names must contain at least one '.' (period)
1597 character (and thus at least two elements).
1600 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1601 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1605 Note that the hyphen ('-') character is allowed in bus names but
1606 not in interface names.
1610 Like <link linkend="message-protocol-names-interface">interface
1611 names</link>, well-known bus names should start with the
1612 reversed DNS domain name of the author of the interface (in
1613 lower-case), and it is conventional for the rest of the well-known
1614 bus name to consist of words run together, with initial
1615 capital letters. As with interface names, including a version
1616 number in well-known bus names is a good idea; it's possible to
1617 have the well-known bus name for more than one version
1618 simultaneously if backwards compatibility is required.
1622 If a well-known bus name implies the presence of a "main" interface,
1623 that "main" interface is often given the same name as
1624 the well-known bus name, and situated at the corresponding object
1625 path. For instance, if the owner of <literal>example.com</literal>
1626 is developing a D-Bus API for a music player, they might define
1627 that any application that takes the well-known name
1628 <literal>com.example.MusicPlayer1</literal> should have an object
1629 at the object path <literal>/com/example/MusicPlayer1</literal>
1630 which implements the interface
1631 <literal>com.example.MusicPlayer1</literal>.
1634 <sect3 id="message-protocol-names-member">
1635 <title>Member names</title>
1637 Member (i.e. method or signal) names:
1639 <listitem><para>Must only contain the ASCII characters
1640 "[A-Z][a-z][0-9]_" and may not begin with a
1641 digit.</para></listitem>
1642 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1643 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1644 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1649 It is conventional for member names on D-Bus to consist of
1650 capitalized words with no punctuation ("camel-case").
1651 Method names should usually be verbs, such as
1652 <literal>GetItems</literal>, and signal names should usually be
1653 a description of an event, such as <literal>ItemsChanged</literal>.
1656 <sect3 id="message-protocol-names-error">
1657 <title>Error names</title>
1659 Error names have the same restrictions as interface names.
1663 Error names have the same naming conventions as interface
1664 names, and often contain <literal>.Error.</literal>; for instance,
1665 the owner of <literal>example.com</literal> might define the
1666 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1667 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1668 The errors defined by D-Bus itself, such as
1669 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1675 <sect2 id="message-protocol-types">
1676 <title>Message Types</title>
1678 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1679 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1680 This section describes these conventions.
1682 <sect3 id="message-protocol-types-method">
1683 <title>Method Calls</title>
1685 Some messages invoke an operation on a remote object. These are
1686 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1687 messages map naturally to methods on objects in a typical program.
1690 A method call message is required to have a <literal>MEMBER</literal> header field
1691 indicating the name of the method. Optionally, the message has an
1692 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1693 Including the <literal>INTERFACE</literal> in all method call
1694 messages is strongly recommended.
1697 In the absence of an <literal>INTERFACE</literal> field, if two
1698 or more interfaces on the same object have a method with the same
1699 name, it is undefined which of those methods will be invoked.
1700 Implementations may choose to either return an error, or deliver the
1701 message as though it had an arbitrary one of those interfaces.
1704 In some situations (such as the well-known system bus), messages
1705 are filtered through an access-control list external to the
1706 remote object implementation. If that filter rejects certain
1707 messages by matching their interface, or accepts only messages
1708 to specific interfaces, it must also reject messages that have no
1709 <literal>INTERFACE</literal>: otherwise, malicious
1710 applications could use this to bypass the filter.
1713 Method call messages also include a <literal>PATH</literal> field
1714 indicating the object to invoke the method on. If the call is passing
1715 through a message bus, the message will also have a
1716 <literal>DESTINATION</literal> field giving the name of the connection
1717 to receive the message.
1720 When an application handles a method call message, it is required to
1721 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1722 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1723 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1726 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1727 are the return value(s) or "out parameters" of the method call.
1728 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1729 and the call fails; no return value will be provided. It makes
1730 no sense to send multiple replies to the same method call.
1733 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1734 reply is required, so the caller will know the method
1735 was successfully processed.
1738 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1742 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1743 then as an optimization the application receiving the method
1744 call may choose to omit the reply message (regardless of
1745 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1746 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1747 flag and reply anyway.
1750 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1751 destination name does not exist then a program to own the destination
1752 name will be started before the message is delivered. The message
1753 will be held until the new program is successfully started or has
1754 failed to start; in case of failure, an error will be returned. This
1755 flag is only relevant in the context of a message bus, it is ignored
1756 during one-to-one communication with no intermediate bus.
1758 <sect4 id="message-protocol-types-method-apis">
1759 <title>Mapping method calls to native APIs</title>
1761 APIs for D-Bus may map method calls to a method call in a specific
1762 programming language, such as C++, or may map a method call written
1763 in an IDL to a D-Bus message.
1766 In APIs of this nature, arguments to a method are often termed "in"
1767 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1768 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1769 "inout" arguments, which are both sent and received, i.e. the caller
1770 passes in a value which is modified. Mapped to D-Bus, an "inout"
1771 argument is equivalent to an "in" argument, followed by an "out"
1772 argument. You can't pass things "by reference" over the wire, so
1773 "inout" is purely an illusion of the in-process API.
1776 Given a method with zero or one return values, followed by zero or more
1777 arguments, where each argument may be "in", "out", or "inout", the
1778 caller constructs a message by appending each "in" or "inout" argument,
1779 in order. "out" arguments are not represented in the caller's message.
1782 The recipient constructs a reply by appending first the return value
1783 if any, then each "out" or "inout" argument, in order.
1784 "in" arguments are not represented in the reply message.
1787 Error replies are normally mapped to exceptions in languages that have
1791 In converting from native APIs to D-Bus, it is perhaps nice to
1792 map D-Bus naming conventions ("FooBar") to native conventions
1793 such as "fooBar" or "foo_bar" automatically. This is OK
1794 as long as you can say that the native API is one that
1795 was specifically written for D-Bus. It makes the most sense
1796 when writing object implementations that will be exported
1797 over the bus. Object proxies used to invoke remote D-Bus
1798 objects probably need the ability to call any D-Bus method,
1799 and thus a magic name mapping like this could be a problem.
1802 This specification doesn't require anything of native API bindings;
1803 the preceding is only a suggested convention for consistency
1809 <sect3 id="message-protocol-types-signal">
1810 <title>Signal Emission</title>
1812 Unlike method calls, signal emissions have no replies.
1813 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1814 It must have three header fields: <literal>PATH</literal> giving the object
1815 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1816 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1817 for signals, though it is optional for method calls.
1821 <sect3 id="message-protocol-types-errors">
1822 <title>Errors</title>
1824 Messages of type <literal>ERROR</literal> are most commonly replies
1825 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1826 to any kind of message. The message bus for example
1827 will return an <literal>ERROR</literal> in reply to a signal emission if
1828 the bus does not have enough memory to send the signal.
1831 An <literal>ERROR</literal> may have any arguments, but if the first
1832 argument is a <literal>STRING</literal>, it must be an error message.
1833 The error message may be logged or shown to the user
1838 <sect3 id="message-protocol-types-notation">
1839 <title>Notation in this document</title>
1841 This document uses a simple pseudo-IDL to describe particular method
1842 calls and signals. Here is an example of a method call:
1844 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1845 out UINT32 resultcode)
1847 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1848 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1849 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1850 characters so it's known that the last part of the name in
1851 the "IDL" is the member name.
1854 In C++ that might end up looking like this:
1856 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1857 unsigned int flags);
1859 or equally valid, the return value could be done as an argument:
1861 void org::freedesktop::DBus::StartServiceByName (const char *name,
1863 unsigned int *resultcode);
1865 It's really up to the API designer how they want to make
1866 this look. You could design an API where the namespace wasn't used
1867 in C++, using STL or Qt, using varargs, or whatever you wanted.
1870 Signals are written as follows:
1872 org.freedesktop.DBus.NameLost (STRING name)
1874 Signals don't specify "in" vs. "out" because only
1875 a single direction is possible.
1878 It isn't especially encouraged to use this lame pseudo-IDL in actual
1879 API implementations; you might use the native notation for the
1880 language you're using, or you might use COM or CORBA IDL, for example.
1885 <sect2 id="message-protocol-handling-invalid">
1886 <title>Invalid Protocol and Spec Extensions</title>
1889 For security reasons, the D-Bus protocol should be strictly parsed and
1890 validated, with the exception of defined extension points. Any invalid
1891 protocol or spec violations should result in immediately dropping the
1892 connection without notice to the other end. Exceptions should be
1893 carefully considered, e.g. an exception may be warranted for a
1894 well-understood idiosyncrasy of a widely-deployed implementation. In
1895 cases where the other end of a connection is 100% trusted and known to
1896 be friendly, skipping validation for performance reasons could also make
1897 sense in certain cases.
1901 Generally speaking violations of the "must" requirements in this spec
1902 should be considered possible attempts to exploit security, and violations
1903 of the "should" suggestions should be considered legitimate (though perhaps
1904 they should generate an error in some cases).
1908 The following extension points are built in to D-Bus on purpose and must
1909 not be treated as invalid protocol. The extension points are intended
1910 for use by future versions of this spec, they are not intended for third
1911 parties. At the moment, the only way a third party could extend D-Bus
1912 without breaking interoperability would be to introduce a way to negotiate new
1913 feature support as part of the auth protocol, using EXTENSION_-prefixed
1914 commands. There is not yet a standard way to negotiate features.
1918 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1919 commands result in an ERROR rather than a disconnect. This enables
1920 future extensions to the protocol. Commands starting with EXTENSION_ are
1921 reserved for third parties.
1926 The authentication protocol supports pluggable auth mechanisms.
1931 The address format (see <xref linkend="addresses"/>) supports new
1937 Messages with an unknown type (something other than
1938 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1939 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1940 Unknown-type messages must still be well-formed in the same way
1941 as the known messages, however. They still have the normal
1947 Header fields with an unknown or unexpected field code must be ignored,
1948 though again they must still be well-formed.
1953 New standard interfaces (with new methods and signals) can of course be added.
1963 <sect1 id="auth-protocol">
1964 <title>Authentication Protocol</title>
1966 Before the flow of messages begins, two applications must
1967 authenticate. A simple plain-text protocol is used for
1968 authentication; this protocol is a SASL profile, and maps fairly
1969 directly from the SASL specification. The message encoding is
1970 NOT used here, only plain text messages.
1973 In examples, "C:" and "S:" indicate lines sent by the client and
1974 server respectively.
1976 <sect2 id="auth-protocol-overview">
1977 <title>Protocol Overview</title>
1979 The protocol is a line-based protocol, where each line ends with
1980 \r\n. Each line begins with an all-caps ASCII command name containing
1981 only the character range [A-Z_], a space, then any arguments for the
1982 command, then the \r\n ending the line. The protocol is
1983 case-sensitive. All bytes must be in the ASCII character set.
1985 Commands from the client to the server are as follows:
1988 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1989 <listitem><para>CANCEL</para></listitem>
1990 <listitem><para>BEGIN</para></listitem>
1991 <listitem><para>DATA <data in hex encoding></para></listitem>
1992 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1993 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1996 From server to client are as follows:
1999 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
2000 <listitem><para>OK <GUID in hex></para></listitem>
2001 <listitem><para>DATA <data in hex encoding></para></listitem>
2002 <listitem><para>ERROR</para></listitem>
2003 <listitem><para>AGREE_UNIX_FD</para></listitem>
2007 Unofficial extensions to the command set must begin with the letters
2008 "EXTENSION_", to avoid conflicts with future official commands.
2009 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2012 <sect2 id="auth-nul-byte">
2013 <title>Special credentials-passing nul byte</title>
2015 Immediately after connecting to the server, the client must send a
2016 single nul byte. This byte may be accompanied by credentials
2017 information on some operating systems that use sendmsg() with
2018 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2019 sockets. However, the nul byte must be sent even on other kinds of
2020 socket, and even on operating systems that do not require a byte to be
2021 sent in order to transmit credentials. The text protocol described in
2022 this document begins after the single nul byte. If the first byte
2023 received from the client is not a nul byte, the server may disconnect
2027 A nul byte in any context other than the initial byte is an error;
2028 the protocol is ASCII-only.
2031 The credentials sent along with the nul byte may be used with the
2032 SASL mechanism EXTERNAL.
2035 <sect2 id="auth-command-auth">
2036 <title>AUTH command</title>
2038 If an AUTH command has no arguments, it is a request to list
2039 available mechanisms. The server must respond with a REJECTED
2040 command listing the mechanisms it understands, or with an error.
2043 If an AUTH command specifies a mechanism, and the server supports
2044 said mechanism, the server should begin exchanging SASL
2045 challenge-response data with the client using DATA commands.
2048 If the server does not support the mechanism given in the AUTH
2049 command, it must send either a REJECTED command listing the mechanisms
2050 it does support, or an error.
2053 If the [initial-response] argument is provided, it is intended for use
2054 with mechanisms that have no initial challenge (or an empty initial
2055 challenge), as if it were the argument to an initial DATA command. If
2056 the selected mechanism has an initial challenge and [initial-response]
2057 was provided, the server should reject authentication by sending
2061 If authentication succeeds after exchanging DATA commands,
2062 an OK command must be sent to the client.
2065 The first octet received by the server after the \r\n of the BEGIN
2066 command from the client must be the first octet of the
2067 authenticated/encrypted stream of D-Bus messages.
2070 If BEGIN is received by the server, the first octet received
2071 by the client after the \r\n of the OK command must be the
2072 first octet of the authenticated/encrypted stream of D-Bus
2076 <sect2 id="auth-command-cancel">
2077 <title>CANCEL Command</title>
2079 At any time up to sending the BEGIN command, the client may send a
2080 CANCEL command. On receiving the CANCEL command, the server must
2081 send a REJECTED command and abort the current authentication
2085 <sect2 id="auth-command-data">
2086 <title>DATA Command</title>
2088 The DATA command may come from either client or server, and simply
2089 contains a hex-encoded block of data to be interpreted
2090 according to the SASL mechanism in use.
2093 Some SASL mechanisms support sending an "empty string";
2094 FIXME we need some way to do this.
2097 <sect2 id="auth-command-begin">
2098 <title>BEGIN Command</title>
2100 The BEGIN command acknowledges that the client has received an
2101 OK command from the server, and that the stream of messages
2105 The first octet received by the server after the \r\n of the BEGIN
2106 command from the client must be the first octet of the
2107 authenticated/encrypted stream of D-Bus messages.
2110 <sect2 id="auth-command-rejected">
2111 <title>REJECTED Command</title>
2113 The REJECTED command indicates that the current authentication
2114 exchange has failed, and further exchange of DATA is inappropriate.
2115 The client would normally try another mechanism, or try providing
2116 different responses to challenges.
2118 Optionally, the REJECTED command has a space-separated list of
2119 available auth mechanisms as arguments. If a server ever provides
2120 a list of supported mechanisms, it must provide the same list
2121 each time it sends a REJECTED message. Clients are free to
2122 ignore all lists received after the first.
2125 <sect2 id="auth-command-ok">
2126 <title>OK Command</title>
2128 The OK command indicates that the client has been
2129 authenticated. The client may now proceed with negotiating
2130 Unix file descriptor passing. To do that it shall send
2131 NEGOTIATE_UNIX_FD to the server.
2134 Otherwise, the client must respond to the OK command by
2135 sending a BEGIN command, followed by its stream of messages,
2136 or by disconnecting. The server must not accept additional
2137 commands using this protocol after the BEGIN command has been
2138 received. Further communication will be a stream of D-Bus
2139 messages (optionally encrypted, as negotiated) rather than
2143 If a client sends BEGIN the first octet received by the client
2144 after the \r\n of the OK command must be the first octet of
2145 the authenticated/encrypted stream of D-Bus messages.
2148 The OK command has one argument, which is the GUID of the server.
2149 See <xref linkend="addresses"/> for more on server GUIDs.
2152 <sect2 id="auth-command-error">
2153 <title>ERROR Command</title>
2155 The ERROR command indicates that either server or client did not
2156 know a command, does not accept the given command in the current
2157 context, or did not understand the arguments to the command. This
2158 allows the protocol to be extended; a client or server can send a
2159 command present or permitted only in new protocol versions, and if
2160 an ERROR is received instead of an appropriate response, fall back
2161 to using some other technique.
2164 If an ERROR is sent, the server or client that sent the
2165 error must continue as if the command causing the ERROR had never been
2166 received. However, the the server or client receiving the error
2167 should try something other than whatever caused the error;
2168 if only canceling/rejecting the authentication.
2171 If the D-Bus protocol changes incompatibly at some future time,
2172 applications implementing the new protocol would probably be able to
2173 check for support of the new protocol by sending a new command and
2174 receiving an ERROR from applications that don't understand it. Thus the
2175 ERROR feature of the auth protocol is an escape hatch that lets us
2176 negotiate extensions or changes to the D-Bus protocol in the future.
2179 <sect2 id="auth-command-negotiate-unix-fd">
2180 <title>NEGOTIATE_UNIX_FD Command</title>
2182 The NEGOTIATE_UNIX_FD command indicates that the client
2183 supports Unix file descriptor passing. This command may only
2184 be sent after the connection is authenticated, i.e. after OK
2185 was received by the client. This command may only be sent on
2186 transports that support Unix file descriptor passing.
2189 On receiving NEGOTIATE_UNIX_FD the server must respond with
2190 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2191 the transport chosen supports Unix file descriptor passing and
2192 the server supports this feature. It shall respond the latter
2193 if the transport does not support Unix file descriptor
2194 passing, the server does not support this feature, or the
2195 server decides not to enable file descriptor passing due to
2196 security or other reasons.
2199 <sect2 id="auth-command-agree-unix-fd">
2200 <title>AGREE_UNIX_FD Command</title>
2202 The AGREE_UNIX_FD command indicates that the server supports
2203 Unix file descriptor passing. This command may only be sent
2204 after the connection is authenticated, and the client sent
2205 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2206 command may only be sent on transports that support Unix file
2210 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2211 followed by its stream of messages, or by disconnecting. The
2212 server must not accept additional commands using this protocol
2213 after the BEGIN command has been received. Further
2214 communication will be a stream of D-Bus messages (optionally
2215 encrypted, as negotiated) rather than this protocol.
2218 <sect2 id="auth-command-future">
2219 <title>Future Extensions</title>
2221 Future extensions to the authentication and negotiation
2222 protocol are possible. For that new commands may be
2223 introduced. If a client or server receives an unknown command
2224 it shall respond with ERROR and not consider this fatal. New
2225 commands may be introduced both before, and after
2226 authentication, i.e. both before and after the OK command.
2229 <sect2 id="auth-examples">
2230 <title>Authentication examples</title>
2234 <title>Example of successful magic cookie authentication</title>
2236 (MAGIC_COOKIE is a made up mechanism)
2238 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2244 <title>Example of finding out mechanisms then picking one</title>
2247 S: REJECTED KERBEROS_V4 SKEY
2248 C: AUTH SKEY 7ab83f32ee
2249 S: DATA 8799cabb2ea93e
2250 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2256 <title>Example of client sends unknown command then falls back to regular auth</title>
2260 C: AUTH MAGIC_COOKIE 3736343435313230333039
2266 <title>Example of server doesn't support initial auth mechanism</title>
2268 C: AUTH MAGIC_COOKIE 3736343435313230333039
2269 S: REJECTED KERBEROS_V4 SKEY
2270 C: AUTH SKEY 7ab83f32ee
2271 S: DATA 8799cabb2ea93e
2272 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2278 <title>Example of wrong password or the like followed by successful retry</title>
2280 C: AUTH MAGIC_COOKIE 3736343435313230333039
2281 S: REJECTED KERBEROS_V4 SKEY
2282 C: AUTH SKEY 7ab83f32ee
2283 S: DATA 8799cabb2ea93e
2284 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2286 C: AUTH SKEY 7ab83f32ee
2287 S: DATA 8799cabb2ea93e
2288 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2294 <title>Example of skey cancelled and restarted</title>
2296 C: AUTH MAGIC_COOKIE 3736343435313230333039
2297 S: REJECTED KERBEROS_V4 SKEY
2298 C: AUTH SKEY 7ab83f32ee
2299 S: DATA 8799cabb2ea93e
2302 C: AUTH SKEY 7ab83f32ee
2303 S: DATA 8799cabb2ea93e
2304 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2310 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2312 (MAGIC_COOKIE is a made up mechanism)
2314 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2316 C: NEGOTIATE_UNIX_FD
2322 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2324 (MAGIC_COOKIE is a made up mechanism)
2326 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2328 C: NEGOTIATE_UNIX_FD
2335 <sect2 id="auth-states">
2336 <title>Authentication state diagrams</title>
2339 This section documents the auth protocol in terms of
2340 a state machine for the client and the server. This is
2341 probably the most robust way to implement the protocol.
2344 <sect3 id="auth-states-client">
2345 <title>Client states</title>
2348 To more precisely describe the interaction between the
2349 protocol state machine and the authentication mechanisms the
2350 following notation is used: MECH(CHALL) means that the
2351 server challenge CHALL was fed to the mechanism MECH, which
2357 CONTINUE(RESP) means continue the auth conversation
2358 and send RESP as the response to the server;
2364 OK(RESP) means that after sending RESP to the server
2365 the client side of the auth conversation is finished
2366 and the server should return "OK";
2372 ERROR means that CHALL was invalid and could not be
2378 Both RESP and CHALL may be empty.
2382 The Client starts by getting an initial response from the
2383 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2384 the mechanism did not provide an initial response. If the
2385 mechanism returns CONTINUE, the client starts in state
2386 <emphasis>WaitingForData</emphasis>, if the mechanism
2387 returns OK the client starts in state
2388 <emphasis>WaitingForOK</emphasis>.
2392 The client should keep track of available mechanisms and
2393 which it mechanisms it has already attempted. This list is
2394 used to decide which AUTH command to send. When the list is
2395 exhausted, the client should give up and close the
2400 <title><emphasis>WaitingForData</emphasis></title>
2408 MECH(CHALL) returns CONTINUE(RESP) → send
2410 <emphasis>WaitingForData</emphasis>
2414 MECH(CHALL) returns OK(RESP) → send DATA
2415 RESP, goto <emphasis>WaitingForOK</emphasis>
2419 MECH(CHALL) returns ERROR → send ERROR
2420 [msg], goto <emphasis>WaitingForData</emphasis>
2428 Receive REJECTED [mechs] →
2429 send AUTH [next mech], goto
2430 WaitingForData or <emphasis>WaitingForOK</emphasis>
2435 Receive ERROR → send
2437 <emphasis>WaitingForReject</emphasis>
2442 Receive OK → send
2443 BEGIN, terminate auth
2444 conversation, authenticated
2449 Receive anything else → send
2451 <emphasis>WaitingForData</emphasis>
2459 <title><emphasis>WaitingForOK</emphasis></title>
2464 Receive OK → send BEGIN, terminate auth
2465 conversation, <emphasis>authenticated</emphasis>
2470 Receive REJECTED [mechs] → send AUTH [next mech],
2471 goto <emphasis>WaitingForData</emphasis> or
2472 <emphasis>WaitingForOK</emphasis>
2478 Receive DATA → send CANCEL, goto
2479 <emphasis>WaitingForReject</emphasis>
2485 Receive ERROR → send CANCEL, goto
2486 <emphasis>WaitingForReject</emphasis>
2492 Receive anything else → send ERROR, goto
2493 <emphasis>WaitingForOK</emphasis>
2501 <title><emphasis>WaitingForReject</emphasis></title>
2506 Receive REJECTED [mechs] → send AUTH [next mech],
2507 goto <emphasis>WaitingForData</emphasis> or
2508 <emphasis>WaitingForOK</emphasis>
2514 Receive anything else → terminate auth
2515 conversation, disconnect
2524 <sect3 id="auth-states-server">
2525 <title>Server states</title>
2528 For the server MECH(RESP) means that the client response
2529 RESP was fed to the the mechanism MECH, which returns one of
2534 CONTINUE(CHALL) means continue the auth conversation and
2535 send CHALL as the challenge to the client;
2541 OK means that the client has been successfully
2548 REJECTED means that the client failed to authenticate or
2549 there was an error in RESP.
2554 The server starts out in state
2555 <emphasis>WaitingForAuth</emphasis>. If the client is
2556 rejected too many times the server must disconnect the
2561 <title><emphasis>WaitingForAuth</emphasis></title>
2567 Receive AUTH → send REJECTED [mechs], goto
2568 <emphasis>WaitingForAuth</emphasis>
2574 Receive AUTH MECH RESP
2578 MECH not valid mechanism → send REJECTED
2580 <emphasis>WaitingForAuth</emphasis>
2584 MECH(RESP) returns CONTINUE(CHALL) → send
2586 <emphasis>WaitingForData</emphasis>
2590 MECH(RESP) returns OK → send OK, goto
2591 <emphasis>WaitingForBegin</emphasis>
2595 MECH(RESP) returns REJECTED → send REJECTED
2597 <emphasis>WaitingForAuth</emphasis>
2605 Receive BEGIN → terminate
2606 auth conversation, disconnect
2612 Receive ERROR → send REJECTED [mechs], goto
2613 <emphasis>WaitingForAuth</emphasis>
2619 Receive anything else → send
2621 <emphasis>WaitingForAuth</emphasis>
2630 <title><emphasis>WaitingForData</emphasis></title>
2638 MECH(RESP) returns CONTINUE(CHALL) → send
2640 <emphasis>WaitingForData</emphasis>
2644 MECH(RESP) returns OK → send OK, goto
2645 <emphasis>WaitingForBegin</emphasis>
2649 MECH(RESP) returns REJECTED → send REJECTED
2651 <emphasis>WaitingForAuth</emphasis>
2659 Receive BEGIN → terminate auth conversation,
2666 Receive CANCEL → send REJECTED [mechs], goto
2667 <emphasis>WaitingForAuth</emphasis>
2673 Receive ERROR → send REJECTED [mechs], goto
2674 <emphasis>WaitingForAuth</emphasis>
2680 Receive anything else → send ERROR, goto
2681 <emphasis>WaitingForData</emphasis>
2689 <title><emphasis>WaitingForBegin</emphasis></title>
2694 Receive BEGIN → terminate auth conversation,
2695 client authenticated
2701 Receive CANCEL → send REJECTED [mechs], goto
2702 <emphasis>WaitingForAuth</emphasis>
2708 Receive ERROR → send REJECTED [mechs], goto
2709 <emphasis>WaitingForAuth</emphasis>
2715 Receive anything else → send ERROR, goto
2716 <emphasis>WaitingForBegin</emphasis>
2726 <sect2 id="auth-mechanisms">
2727 <title>Authentication mechanisms</title>
2729 This section describes some new authentication mechanisms.
2730 D-Bus also allows any standard SASL mechanism of course.
2732 <sect3 id="auth-mechanisms-sha">
2733 <title>DBUS_COOKIE_SHA1</title>
2735 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2736 has the ability to read a private file owned by the user being
2737 authenticated. If the client can prove that it has access to a secret
2738 cookie stored in this file, then the client is authenticated.
2739 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2743 Throughout this description, "hex encoding" must output the digits
2744 from a to f in lower-case; the digits A to F must not be used
2745 in the DBUS_COOKIE_SHA1 mechanism.
2748 Authentication proceeds as follows:
2752 The client sends the username it would like to authenticate
2758 The server sends the name of its "cookie context" (see below); a
2759 space character; the integer ID of the secret cookie the client
2760 must demonstrate knowledge of; a space character; then a
2761 randomly-generated challenge string, all of this hex-encoded into
2767 The client locates the cookie and generates its own
2768 randomly-generated challenge string. The client then concatenates
2769 the server's decoded challenge, a ":" character, its own challenge,
2770 another ":" character, and the cookie. It computes the SHA-1 hash
2771 of this composite string as a hex digest. It concatenates the
2772 client's challenge string, a space character, and the SHA-1 hex
2773 digest, hex-encodes the result and sends it back to the server.
2778 The server generates the same concatenated string used by the
2779 client and computes its SHA-1 hash. It compares the hash with
2780 the hash received from the client; if the two hashes match, the
2781 client is authenticated.
2787 Each server has a "cookie context," which is a name that identifies a
2788 set of cookies that apply to that server. A sample context might be
2789 "org_freedesktop_session_bus". Context names must be valid ASCII,
2790 nonzero length, and may not contain the characters slash ("/"),
2791 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2792 tab ("\t"), or period ("."). There is a default context,
2793 "org_freedesktop_general" that's used by servers that do not specify
2797 Cookies are stored in a user's home directory, in the directory
2798 <filename>~/.dbus-keyrings/</filename>. This directory must
2799 not be readable or writable by other users. If it is,
2800 clients and servers must ignore it. The directory
2801 contains cookie files named after the cookie context.
2804 A cookie file contains one cookie per line. Each line
2805 has three space-separated fields:
2809 The cookie ID number, which must be a non-negative integer and
2810 may not be used twice in the same file.
2815 The cookie's creation time, in UNIX seconds-since-the-epoch
2821 The cookie itself, a hex-encoded random block of bytes. The cookie
2822 may be of any length, though obviously security increases
2823 as the length increases.
2829 Only server processes modify the cookie file.
2830 They must do so with this procedure:
2834 Create a lockfile name by appending ".lock" to the name of the
2835 cookie file. The server should attempt to create this file
2836 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2837 fails, the lock fails. Servers should retry for a reasonable
2838 period of time, then they may choose to delete an existing lock
2839 to keep users from having to manually delete a stale
2840 lock. <footnote><para>Lockfiles are used instead of real file
2841 locking <literal>fcntl()</literal> because real locking
2842 implementations are still flaky on network
2843 filesystems.</para></footnote>
2848 Once the lockfile has been created, the server loads the cookie
2849 file. It should then delete any cookies that are old (the
2850 timeout can be fairly short), or more than a reasonable
2851 time in the future (so that cookies never accidentally
2852 become permanent, if the clock was set far into the future
2853 at some point). If no recent keys remain, the
2854 server may generate a new key.
2859 The pruned and possibly added-to cookie file
2860 must be resaved atomically (using a temporary
2861 file which is rename()'d).
2866 The lock must be dropped by deleting the lockfile.
2872 Clients need not lock the file in order to load it,
2873 because servers are required to save the file atomically.
2878 <sect1 id="addresses">
2879 <title>Server Addresses</title>
2881 Server addresses consist of a transport name followed by a colon, and
2882 then an optional, comma-separated list of keys and values in the form key=value.
2883 Each value is escaped.
2887 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2888 Which is the address to a unix socket with the path /tmp/dbus-test.
2891 Value escaping is similar to URI escaping but simpler.
2895 The set of optionally-escaped bytes is:
2896 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2897 <emphasis>byte</emphasis> (note, not character) which is not in the
2898 set of optionally-escaped bytes must be replaced with an ASCII
2899 percent (<literal>%</literal>) and the value of the byte in hex.
2900 The hex value must always be two digits, even if the first digit is
2901 zero. The optionally-escaped bytes may be escaped if desired.
2906 To unescape, append each byte in the value; if a byte is an ASCII
2907 percent (<literal>%</literal>) character then append the following
2908 hex value instead. It is an error if a <literal>%</literal> byte
2909 does not have two hex digits following. It is an error if a
2910 non-optionally-escaped byte is seen unescaped.
2914 The set of optionally-escaped bytes is intended to preserve address
2915 readability and convenience.
2919 A server may specify a key-value pair with the key <literal>guid</literal>
2920 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2921 describes the format of the <literal>guid</literal> field. If present,
2922 this UUID may be used to distinguish one server address from another. A
2923 server should use a different UUID for each address it listens on. For
2924 example, if a message bus daemon offers both UNIX domain socket and TCP
2925 connections, but treats clients the same regardless of how they connect,
2926 those two connections are equivalent post-connection but should have
2927 distinct UUIDs to distinguish the kinds of connection.
2931 The intent of the address UUID feature is to allow a client to avoid
2932 opening multiple identical connections to the same server, by allowing the
2933 client to check whether an address corresponds to an already-existing
2934 connection. Comparing two addresses is insufficient, because addresses
2935 can be recycled by distinct servers, and equivalent addresses may look
2936 different if simply compared as strings (for example, the host in a TCP
2937 address can be given as an IP address or as a hostname).
2941 Note that the address key is <literal>guid</literal> even though the
2942 rest of the API and documentation says "UUID," for historical reasons.
2946 [FIXME clarify if attempting to connect to each is a requirement
2947 or just a suggestion]
2948 When connecting to a server, multiple server addresses can be
2949 separated by a semi-colon. The library will then try to connect
2950 to the first address and if that fails, it'll try to connect to
2951 the next one specified, and so forth. For example
2952 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2956 Some addresses are <firstterm>connectable</firstterm>. A connectable
2957 address is one containing enough information for a client to connect
2958 to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
2959 is a connectable address. It is not necessarily possible to listen
2960 on every connectable address: for instance, it is not possible to
2961 listen on a <literal>unixexec:</literal> address.
2965 Some addresses are <firstterm>listenable</firstterm>. A listenable
2966 address is one containing enough information for a server to listen on
2967 it, producing a connectable address (which may differ from the
2968 original address). Many listenable addresses are not connectable:
2969 for instance, <literal>tcp:host=127.0.0.1</literal>
2970 is listenable, but not connectable (because it does not specify
2975 Listening on an address that is not connectable will result in a
2976 connectable address that is not the same as the listenable address.
2977 For instance, listening on <literal>tcp:host=127.0.0.1</literal>
2978 might result in the connectable address
2979 <literal>tcp:host=127.0.0.1,port=30958</literal>,
2980 or listening on <literal>unix:tmpdir=/tmp</literal>
2981 might result in the connectable address
2982 <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>.
2986 <sect1 id="transports">
2987 <title>Transports</title>
2989 [FIXME we need to specify in detail each transport and its possible arguments]
2991 Current transports include: unix domain sockets (including
2992 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
2993 using in-process pipes. Future possible transports include one that
2994 tunnels over X11 protocol.
2997 <sect2 id="transports-unix-domain-sockets">
2998 <title>Unix Domain Sockets</title>
3000 Unix domain sockets can be either paths in the file system or on Linux
3001 kernels, they can be abstract which are similar to paths but
3002 do not show up in the file system.
3006 When a socket is opened by the D-Bus library it truncates the path
3007 name right before the first trailing Nul byte. This is true for both
3008 normal paths and abstract paths. Note that this is a departure from
3009 previous versions of D-Bus that would create sockets with a fixed
3010 length path name. Names which were shorter than the fixed length
3011 would be padded by Nul bytes.
3014 Unix domain sockets are not available on Windows.
3017 Unix addresses that specify <literal>path</literal> or
3018 <literal>abstract</literal> are both listenable and connectable.
3019 Unix addresses that specify <literal>tmpdir</literal> are only
3020 listenable: the corresponding connectable address will specify
3021 either <literal>path</literal> or <literal>abstract</literal>.
3023 <sect3 id="transports-unix-domain-sockets-addresses">
3024 <title>Server Address Format</title>
3026 Unix domain socket addresses are identified by the "unix:" prefix
3027 and support the following key/value pairs:
3034 <entry>Values</entry>
3035 <entry>Description</entry>
3041 <entry>(path)</entry>
3042 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
3045 <entry>tmpdir</entry>
3046 <entry>(path)</entry>
3047 <entry>temporary directory in which a socket file with a random file name starting with 'dbus-' will be created by the server. This key can only be used in server addresses, not in client addresses. If set, the "path" and "abstract" key must not be set.</entry>
3050 <entry>abstract</entry>
3051 <entry>(string)</entry>
3052 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tmpdir" key must not be set. This key is only supported on platforms with "abstract Unix sockets", of which Linux is the only known example.</entry>
3058 Exactly one of the keys <literal>path</literal>,
3059 <literal>abstract</literal> or
3060 <literal>tmpdir</literal> must be provided.
3064 <sect2 id="transports-launchd">
3065 <title>launchd</title>
3067 launchd is an open-source server management system that replaces init, inetd
3068 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3069 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3073 launchd allocates a socket and provides it with the unix path through the
3074 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3075 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3076 it through its environment.
3077 Other processes can query for the launchd socket by executing:
3078 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3079 This is normally done by the D-Bus client library so doesn't have to be done
3083 launchd is not available on Microsoft Windows.
3086 launchd addresses are listenable and connectable.
3088 <sect3 id="transports-launchd-addresses">
3089 <title>Server Address Format</title>
3091 launchd addresses are identified by the "launchd:" prefix
3092 and support the following key/value pairs:
3099 <entry>Values</entry>
3100 <entry>Description</entry>
3106 <entry>(environment variable)</entry>
3107 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3113 The <literal>env</literal> key is required.
3117 <sect2 id="transports-systemd">
3118 <title>systemd</title>
3120 systemd is an open-source server management system that
3121 replaces init and inetd on newer Linux systems. It supports
3122 socket activation. The D-Bus systemd transport is used to acquire
3123 socket activation file descriptors from systemd and use them
3124 as D-Bus transport when the current process is spawned by
3125 socket activation from it.
3128 The systemd transport accepts only one or more Unix domain or
3129 TCP streams sockets passed in via socket activation.
3132 The systemd transport is not available on non-Linux operating systems.
3135 The systemd transport defines no parameter keys.
3138 systemd addresses are listenable, but not connectable. The
3139 corresponding connectable address is the <literal>unix</literal>
3140 or <literal>tcp</literal> address of the socket.
3143 <sect2 id="transports-tcp-sockets">
3144 <title>TCP Sockets</title>
3146 The tcp transport provides TCP/IP based connections between clients
3147 located on the same or different hosts.
3150 Using tcp transport without any additional secure authentification mechanismus
3151 over a network is unsecure.
3154 On Windows and most Unix platforms, the TCP stack is unable to transfer
3155 credentials over a TCP connection, so the EXTERNAL authentication
3156 mechanism does not work for this transport.
3159 All <literal>tcp</literal> addresses are listenable.
3160 <literal>tcp</literal> addresses in which both
3161 <literal>host</literal> and <literal>port</literal> are
3162 specified, and <literal>port</literal> is non-zero,
3163 are also connectable.
3165 <sect3 id="transports-tcp-sockets-addresses">
3166 <title>Server Address Format</title>
3168 TCP/IP socket addresses are identified by the "tcp:" prefix
3169 and support the following key/value pairs:
3176 <entry>Values</entry>
3177 <entry>Description</entry>
3183 <entry>(string)</entry>
3184 <entry>DNS name or IP address</entry>
3188 <entry>(string)</entry>
3189 <entry>Used in a listenable address to configure the interface
3190 on which the server will listen: either the IP address of one of
3191 the local machine's interfaces (most commonly <literal>127.0.0.1
3192 </literal>), or a DNS name that resolves to one of those IP
3193 addresses, or '*' to listen on all interfaces simultaneously.
3194 If not specified, the default is the same value as "host".
3199 <entry>(number)</entry>
3200 <entry>The tcp port the server will open. A zero value let the server
3201 choose a free port provided from the underlaying operating system.
3202 libdbus is able to retrieve the real used port from the server.
3206 <entry>family</entry>
3207 <entry>(string)</entry>
3208 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3215 <sect2 id="transports-nonce-tcp-sockets">
3216 <title>Nonce-secured TCP Sockets</title>
3218 The nonce-tcp transport provides a secured TCP transport, using a
3219 simple authentication mechanism to ensure that only clients with read
3220 access to a certain location in the filesystem can connect to the server.
3221 The server writes a secret, the nonce, to a file and an incoming client
3222 connection is only accepted if the client sends the nonce right after
3223 the connect. The nonce mechanism requires no setup and is orthogonal to
3224 the higher-level authentication mechanisms described in the
3225 Authentication section.
3229 On start, the server generates a random 16 byte nonce and writes it
3230 to a file in the user's temporary directory. The nonce file location
3231 is published as part of the server's D-Bus address using the
3232 "noncefile" key-value pair.
3234 After an accept, the server reads 16 bytes from the socket. If the
3235 read bytes do not match the nonce stored in the nonce file, the
3236 server MUST immediately drop the connection.
3237 If the nonce match the received byte sequence, the client is accepted
3238 and the transport behaves like an unsecured tcp transport.
3241 After a successful connect to the server socket, the client MUST read
3242 the nonce from the file published by the server via the noncefile=
3243 key-value pair and send it over the socket. After that, the
3244 transport behaves like an unsecured tcp transport.
3247 All nonce-tcp addresses are listenable. nonce-tcp addresses in which
3248 <literal>host</literal>, <literal>port</literal> and
3249 <literal>noncefile</literal> are all specified,
3250 and <literal>port</literal> is nonzero, are also connectable.
3252 <sect3 id="transports-nonce-tcp-sockets-addresses">
3253 <title>Server Address Format</title>
3255 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3256 and support the following key/value pairs:
3263 <entry>Values</entry>
3264 <entry>Description</entry>
3270 <entry>(string)</entry>
3271 <entry>DNS name or IP address</entry>
3275 <entry>(string)</entry>
3276 <entry>The same as for tcp: addresses
3281 <entry>(number)</entry>
3282 <entry>The tcp port the server will open. A zero value let the server
3283 choose a free port provided from the underlaying operating system.
3284 libdbus is able to retrieve the real used port from the server.
3288 <entry>family</entry>
3289 <entry>(string)</entry>
3290 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3293 <entry>noncefile</entry>
3294 <entry>(path)</entry>
3295 <entry>File location containing the secret.
3296 This is only meaningful in connectable addresses:
3297 a listening D-Bus server that offers this transport
3298 will always create a new nonce file.</entry>
3305 <sect2 id="transports-exec">
3306 <title>Executed Subprocesses on Unix</title>
3308 This transport forks off a process and connects its standard
3309 input and standard output with an anonymous Unix domain
3310 socket. This socket is then used for communication by the
3311 transport. This transport may be used to use out-of-process
3312 forwarder programs as basis for the D-Bus protocol.
3315 The forked process will inherit the standard error output and
3316 process group from the parent process.
3319 Executed subprocesses are not available on Windows.
3322 <literal>unixexec</literal> addresses are connectable, but are not
3325 <sect3 id="transports-exec-addresses">
3326 <title>Server Address Format</title>
3328 Executed subprocess addresses are identified by the "unixexec:" prefix
3329 and support the following key/value pairs:
3336 <entry>Values</entry>
3337 <entry>Description</entry>
3343 <entry>(path)</entry>
3344 <entry>Path of the binary to execute, either an absolute
3345 path or a binary name that is searched for in the default
3346 search path of the OS. This corresponds to the first
3347 argument of execlp(). This key is mandatory.</entry>
3350 <entry>argv0</entry>
3351 <entry>(string)</entry>
3352 <entry>The program name to use when executing the
3353 binary. If omitted the same value as specified for path=
3354 will be used. This corresponds to the second argument of
3358 <entry>argv1, argv2, ...</entry>
3359 <entry>(string)</entry>
3360 <entry>Arguments to pass to the binary. This corresponds
3361 to the third and later arguments of execlp(). If a
3362 specific argvX is not specified no further argvY for Y > X
3363 are taken into account.</entry>
3371 <sect1 id="meta-transports">
3372 <title>Meta Transports</title>
3374 Meta transports are a kind of transport with special enhancements or
3375 behavior. Currently available meta transports include: autolaunch
3378 <sect2 id="meta-transports-autolaunch">
3379 <title>Autolaunch</title>
3380 <para>The autolaunch transport provides a way for dbus clients to autodetect
3381 a running dbus session bus and to autolaunch a session bus if not present.
3384 On Unix, <literal>autolaunch</literal> addresses are connectable,
3388 On Windows, <literal>autolaunch</literal> addresses are both
3389 connectable and listenable.
3392 <sect3 id="meta-transports-autolaunch-addresses">
3393 <title>Server Address Format</title>
3395 Autolaunch addresses uses the "autolaunch:" prefix and support the
3396 following key/value pairs:
3403 <entry>Values</entry>
3404 <entry>Description</entry>
3409 <entry>scope</entry>
3410 <entry>(string)</entry>
3411 <entry>scope of autolaunch (Windows only)
3415 "*install-path" - limit session bus to dbus installation path.
3416 The dbus installation path is determined from the location of
3417 the shared dbus library. If the library is located in a 'bin'
3418 subdirectory the installation root is the directory above,
3419 otherwise the directory where the library lives is taken as
3422 <install-root>/bin/[lib]dbus-1.dll
3423 <install-root>/[lib]dbus-1.dll
3429 "*user" - limit session bus to the recent user.
3434 other values - specify dedicated session bus like "release",
3446 <sect3 id="meta-transports-autolaunch-windows-implementation">
3447 <title>Windows implementation</title>
3449 On start, the server opens a platform specific transport, creates a mutex
3450 and a shared memory section containing the related session bus address.
3451 This mutex will be inspected by the dbus client library to detect a
3452 running dbus session bus. The access to the mutex and the shared memory
3453 section are protected by global locks.
3456 In the recent implementation the autolaunch transport uses a tcp transport
3457 on localhost with a port choosen from the operating system. This detail may
3458 change in the future.
3461 Disclaimer: The recent implementation is in an early state and may not
3462 work in all cirumstances and/or may have security issues. Because of this
3463 the implementation is not documentated yet.
3470 <title>UUIDs</title>
3472 A working D-Bus implementation uses universally-unique IDs in two places.
3473 First, each server address has a UUID identifying the address,
3474 as described in <xref linkend="addresses"/>. Second, each operating
3475 system kernel instance running a D-Bus client or server has a UUID
3476 identifying that kernel, retrieved by invoking the method
3477 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3478 linkend="standard-interfaces-peer"/>).
3481 The term "UUID" in this document is intended literally, i.e. an
3482 identifier that is universally unique. It is not intended to refer to
3483 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3486 The UUID must contain 128 bits of data and be hex-encoded. The
3487 hex-encoded string may not contain hyphens or other non-hex-digit
3488 characters, and it must be exactly 32 characters long. To generate a
3489 UUID, the current reference implementation concatenates 96 bits of random
3490 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3494 It would also be acceptable and probably better to simply generate 128
3495 bits of random data, as long as the random number generator is of high
3496 quality. The timestamp could conceivably help if the random bits are not
3497 very random. With a quality random number generator, collisions are
3498 extremely unlikely even with only 96 bits, so it's somewhat academic.
3501 Implementations should, however, stick to random data for the first 96 bits
3506 <sect1 id="standard-interfaces">
3507 <title>Standard Interfaces</title>
3509 See <xref linkend="message-protocol-types-notation"/> for details on
3510 the notation used in this section. There are some standard interfaces
3511 that may be useful across various D-Bus applications.
3513 <sect2 id="standard-interfaces-peer">
3514 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3516 The <literal>org.freedesktop.DBus.Peer</literal> interface
3519 org.freedesktop.DBus.Peer.Ping ()
3520 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3524 On receipt of the <literal>METHOD_CALL</literal> message
3525 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3526 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3527 usual. It does not matter which object path a ping is sent to. The
3528 reference implementation handles this method automatically.
3531 On receipt of the <literal>METHOD_CALL</literal> message
3532 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3533 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3534 UUID representing the identity of the machine the process is running on.
3535 This UUID must be the same for all processes on a single system at least
3536 until that system next reboots. It should be the same across reboots
3537 if possible, but this is not always possible to implement and is not
3539 It does not matter which object path a GetMachineId is sent to. The
3540 reference implementation handles this method automatically.
3543 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3544 a virtual machine running on a hypervisor, rather than a physical machine.
3545 Basically if two processes see the same UUID, they should also see the same
3546 shared memory, UNIX domain sockets, process IDs, and other features that require
3547 a running OS kernel in common between the processes.
3550 The UUID is often used where other programs might use a hostname. Hostnames
3551 can change without rebooting, however, or just be "localhost" - so the UUID
3555 <xref linkend="uuids"/> explains the format of the UUID.
3559 <sect2 id="standard-interfaces-introspectable">
3560 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3562 This interface has one method:
3564 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3568 Objects instances may implement
3569 <literal>Introspect</literal> which returns an XML description of
3570 the object, including its interfaces (with signals and methods), objects
3571 below it in the object path tree, and its properties.
3574 <xref linkend="introspection-format"/> describes the format of this XML string.
3577 <sect2 id="standard-interfaces-properties">
3578 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3580 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3581 or <firstterm>attributes</firstterm>. These can be exposed via the
3582 <literal>org.freedesktop.DBus.Properties</literal> interface.
3586 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3587 in STRING property_name,
3589 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3590 in STRING property_name,
3592 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3593 out DICT<STRING,VARIANT> props);
3597 It is conventional to give D-Bus properties names consisting of
3598 capitalized words without punctuation ("CamelCase"), like
3599 <link linkend="message-protocol-names-member">member names</link>.
3600 For instance, the GObject property
3601 <literal>connection-status</literal> or the Qt property
3602 <literal>connectionStatus</literal> could be represented on D-Bus
3603 as <literal>ConnectionStatus</literal>.
3606 Strictly speaking, D-Bus property names are not required to follow
3607 the same naming restrictions as member names, but D-Bus property
3608 names that would not be valid member names (in particular,
3609 GObject-style dash-separated property names) can cause interoperability
3610 problems and should be avoided.
3613 The available properties and whether they are writable can be determined
3614 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3615 see <xref linkend="standard-interfaces-introspectable"/>.
3618 An empty string may be provided for the interface name; in this case,
3619 if there are multiple properties on an object with the same name,
3620 the results are undefined (picking one by according to an arbitrary
3621 deterministic rule, or returning an error, are the reasonable
3625 If one or more properties change on an object, the
3626 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3627 signal may be emitted (this signal was added in 0.14):
3631 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3632 DICT<STRING,VARIANT> changed_properties,
3633 ARRAY<STRING> invalidated_properties);
3637 where <literal>changed_properties</literal> is a dictionary
3638 containing the changed properties with the new values and
3639 <literal>invalidated_properties</literal> is an array of
3640 properties that changed but the value is not conveyed.
3643 Whether the <literal>PropertiesChanged</literal> signal is
3644 supported can be determined by calling
3645 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3646 that the signal may be supported for an object but it may
3647 differ how whether and how it is used on a per-property basis
3648 (for e.g. performance or security reasons). Each property (or
3649 the parent interface) must be annotated with the
3650 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3651 annotation to convey this (usually the default value
3652 <literal>true</literal> is sufficient meaning that the
3653 annotation does not need to be used). See <xref
3654 linkend="introspection-format"/> for details on this
3659 <sect2 id="standard-interfaces-objectmanager">
3660 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3662 An API can optionally make use of this interface for one or
3663 more sub-trees of objects. The root of each sub-tree implements
3664 this interface so other applications can get all objects,
3665 interfaces and properties in a single method call. It is
3666 appropriate to use this interface if users of the tree of
3667 objects are expected to be interested in all interfaces of all
3668 objects in the tree; a more granular API should be used if
3669 users of the objects are expected to be interested in a small
3670 subset of the objects, a small subset of their interfaces, or
3674 The method that applications can use to get all objects and
3675 properties is <literal>GetManagedObjects</literal>:
3679 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3683 The return value of this method is a dict whose keys are
3684 object paths. All returned object paths are children of the
3685 object path implementing this interface, i.e. their object
3686 paths start with the ObjectManager's object path plus '/'.
3689 Each value is a dict whose keys are interfaces names. Each
3690 value in this inner dict is the same dict that would be
3691 returned by the <link
3692 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3693 method for that combination of object path and interface. If
3694 an interface has no properties, the empty dict is returned.
3697 Changes are emitted using the following two signals:
3701 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3702 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3703 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3704 ARRAY<STRING> interfaces);
3708 The <literal>InterfacesAdded</literal> signal is emitted when
3709 either a new object is added or when an existing object gains
3710 one or more interfaces. The
3711 <literal>InterfacesRemoved</literal> signal is emitted
3712 whenever an object is removed or it loses one or more
3713 interfaces. The second parameter of the
3714 <literal>InterfacesAdded</literal> signal contains a dict with
3715 the interfaces and properties (if any) that have been added to
3716 the given object path. Similarly, the second parameter of the
3717 <literal>InterfacesRemoved</literal> signal contains an array
3718 of the interfaces that were removed. Note that changes on
3719 properties on existing interfaces are not reported using this
3720 interface - an application should also monitor the existing <link
3721 linkend="standard-interfaces-properties">PropertiesChanged</link>
3722 signal on each object.
3725 Applications SHOULD NOT export objects that are children of an
3726 object (directly or otherwise) implementing this interface but
3727 which are not returned in the reply from the
3728 <literal>GetManagedObjects()</literal> method of this
3729 interface on the given object.
3732 The intent of the <literal>ObjectManager</literal> interface
3733 is to make it easy to write a robust client
3734 implementation. The trivial client implementation only needs
3735 to make two method calls:
3739 org.freedesktop.DBus.AddMatch (bus_proxy,
3740 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3741 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3745 on the message bus and the remote application's
3746 <literal>ObjectManager</literal>, respectively. Whenever a new
3747 remote object is created (or an existing object gains a new
3748 interface), the <literal>InterfacesAdded</literal> signal is
3749 emitted, and since this signal contains all properties for the
3750 interfaces, no calls to the
3751 <literal>org.freedesktop.Properties</literal> interface on the
3752 remote object are needed. Additionally, since the initial
3753 <literal>AddMatch()</literal> rule already includes signal
3754 messages from the newly created child object, no new
3755 <literal>AddMatch()</literal> call is needed.
3760 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3761 interface was added in version 0.17 of the D-Bus
3768 <sect1 id="introspection-format">
3769 <title>Introspection Data Format</title>
3771 As described in <xref linkend="standard-interfaces-introspectable"/>,
3772 objects may be introspected at runtime, returning an XML string
3773 that describes the object. The same XML format may be used in
3774 other contexts as well, for example as an "IDL" for generating
3775 static language bindings.
3778 Here is an example of introspection data:
3780 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3781 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3782 <node name="/com/example/sample_object">
3783 <interface name="com.example.SampleInterface">
3784 <method name="Frobate">
3785 <arg name="foo" type="i" direction="in"/>
3786 <arg name="bar" type="s" direction="out"/>
3787 <arg name="baz" type="a{us}" direction="out"/>
3788 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3790 <method name="Bazify">
3791 <arg name="bar" type="(iiu)" direction="in"/>
3792 <arg name="bar" type="v" direction="out"/>
3794 <method name="Mogrify">
3795 <arg name="bar" type="(iiav)" direction="in"/>
3797 <signal name="Changed">
3798 <arg name="new_value" type="b"/>
3800 <property name="Bar" type="y" access="readwrite"/>
3802 <node name="child_of_sample_object"/>
3803 <node name="another_child_of_sample_object"/>
3808 A more formal DTD and spec needs writing, but here are some quick notes.
3812 Only the root <node> element can omit the node name, as it's
3813 known to be the object that was introspected. If the root
3814 <node> does have a name attribute, it must be an absolute
3815 object path. If child <node> have object paths, they must be
3821 If a child <node> has any sub-elements, then they
3822 must represent a complete introspection of the child.
3823 If a child <node> is empty, then it may or may
3824 not have sub-elements; the child must be introspected
3825 in order to find out. The intent is that if an object
3826 knows that its children are "fast" to introspect
3827 it can go ahead and return their information, but
3828 otherwise it can omit it.
3833 The direction element on <arg> may be omitted,
3834 in which case it defaults to "in" for method calls
3835 and "out" for signals. Signals only allow "out"
3836 so while direction may be specified, it's pointless.
3841 The possible directions are "in" and "out",
3842 unlike CORBA there is no "inout"
3847 The possible property access flags are
3848 "readwrite", "read", and "write"
3853 Multiple interfaces can of course be listed for
3859 The "name" attribute on arguments is optional.
3865 Method, interface, property, and signal elements may have
3866 "annotations", which are generic key/value pairs of metadata.
3867 They are similar conceptually to Java's annotations and C# attributes.
3868 Well-known annotations:
3875 <entry>Values (separated by ,)</entry>
3876 <entry>Description</entry>
3881 <entry>org.freedesktop.DBus.Deprecated</entry>
3882 <entry>true,false</entry>
3883 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3886 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3887 <entry>(string)</entry>
3888 <entry>The C symbol; may be used for methods and interfaces</entry>
3891 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3892 <entry>true,false</entry>
3893 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3896 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3897 <entry>true,invalidates,false</entry>
3900 If set to <literal>false</literal>, the
3901 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3903 linkend="standard-interfaces-properties"/> is not
3904 guaranteed to be emitted if the property changes.
3907 If set to <literal>invalidates</literal> the signal
3908 is emitted but the value is not included in the
3912 If set to <literal>true</literal> the signal is
3913 emitted with the value included.
3916 The value for the annotation defaults to
3917 <literal>true</literal> if the enclosing interface
3918 element does not specify the annotation. Otherwise it
3919 defaults to the value specified in the enclosing
3928 <sect1 id="message-bus">
3929 <title>Message Bus Specification</title>
3930 <sect2 id="message-bus-overview">
3931 <title>Message Bus Overview</title>
3933 The message bus accepts connections from one or more applications.
3934 Once connected, applications can exchange messages with other
3935 applications that are also connected to the bus.
3938 In order to route messages among connections, the message bus keeps a
3939 mapping from names to connections. Each connection has one
3940 unique-for-the-lifetime-of-the-bus name automatically assigned.
3941 Applications may request additional names for a connection. Additional
3942 names are usually "well-known names" such as
3943 "com.example.TextEditor". When a name is bound to a connection,
3944 that connection is said to <firstterm>own</firstterm> the name.
3947 The bus itself owns a special name,
3948 <literal>org.freedesktop.DBus</literal>, with an object
3949 located at <literal>/org/freedesktop/DBus</literal> that
3950 implements the <literal>org.freedesktop.DBus</literal>
3951 interface. This service allows applications to make
3952 administrative requests of the bus itself. For example,
3953 applications can ask the bus to assign a name to a connection.
3956 Each name may have <firstterm>queued owners</firstterm>. When an
3957 application requests a name for a connection and the name is already in
3958 use, the bus will optionally add the connection to a queue waiting for
3959 the name. If the current owner of the name disconnects or releases
3960 the name, the next connection in the queue will become the new owner.
3964 This feature causes the right thing to happen if you start two text
3965 editors for example; the first one may request "com.example.TextEditor",
3966 and the second will be queued as a possible owner of that name. When
3967 the first exits, the second will take over.
3971 Applications may send <firstterm>unicast messages</firstterm> to
3972 a specific recipient or to the message bus itself, or
3973 <firstterm>broadcast messages</firstterm> to all interested recipients.
3974 See <xref linkend="message-bus-routing"/> for details.
3978 <sect2 id="message-bus-names">
3979 <title>Message Bus Names</title>
3981 Each connection has at least one name, assigned at connection time and
3982 returned in response to the
3983 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3984 automatically-assigned name is called the connection's <firstterm>unique
3985 name</firstterm>. Unique names are never reused for two different
3986 connections to the same bus.
3989 Ownership of a unique name is a prerequisite for interaction with
3990 the message bus. It logically follows that the unique name is always
3991 the first name that an application comes to own, and the last
3992 one that it loses ownership of.
3995 Unique connection names must begin with the character ':' (ASCII colon
3996 character); bus names that are not unique names must not begin
3997 with this character. (The bus must reject any attempt by an application
3998 to manually request a name beginning with ':'.) This restriction
3999 categorically prevents "spoofing"; messages sent to a unique name
4000 will always go to the expected connection.
4003 When a connection is closed, all the names that it owns are deleted (or
4004 transferred to the next connection in the queue if any).
4007 A connection can request additional names to be associated with it using
4008 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
4009 linkend="message-protocol-names-bus"/> describes the format of a valid
4010 name. These names can be released again using the
4011 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
4014 <sect3 id="bus-messages-request-name">
4015 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
4019 UINT32 RequestName (in STRING name, in UINT32 flags)
4026 <entry>Argument</entry>
4028 <entry>Description</entry>
4034 <entry>STRING</entry>
4035 <entry>Name to request</entry>
4039 <entry>UINT32</entry>
4040 <entry>Flags</entry>
4050 <entry>Argument</entry>
4052 <entry>Description</entry>
4058 <entry>UINT32</entry>
4059 <entry>Return value</entry>
4066 This method call should be sent to
4067 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4068 assign the given name to the method caller. Each name maintains a
4069 queue of possible owners, where the head of the queue is the primary
4070 or current owner of the name. Each potential owner in the queue
4071 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
4072 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
4073 call. When RequestName is invoked the following occurs:
4077 If the method caller is currently the primary owner of the name,
4078 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
4079 values are updated with the values from the new RequestName call,
4080 and nothing further happens.
4086 If the current primary owner (head of the queue) has
4087 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
4088 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
4089 the caller of RequestName replaces the current primary owner at
4090 the head of the queue and the current primary owner moves to the
4091 second position in the queue. If the caller of RequestName was
4092 in the queue previously its flags are updated with the values from
4093 the new RequestName in addition to moving it to the head of the queue.
4099 If replacement is not possible, and the method caller is
4100 currently in the queue but not the primary owner, its flags are
4101 updated with the values from the new RequestName call.
4107 If replacement is not possible, and the method caller is
4108 currently not in the queue, the method caller is appended to the
4115 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4116 set and is not the primary owner, it is removed from the
4117 queue. This can apply to the previous primary owner (if it
4118 was replaced) or the method caller (if it updated the
4119 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4120 queue, or if it was just added to the queue with that flag set).
4126 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4127 queue," even if another application already in the queue had specified
4128 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4129 that does not allow replacement goes away, and the next primary owner
4130 does allow replacement. In this case, queued items that specified
4131 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4132 automatically replace the new primary owner. In other words,
4133 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4134 time RequestName is called. This is deliberate to avoid an infinite loop
4135 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4136 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4139 The flags argument contains any of the following values logically ORed
4146 <entry>Conventional Name</entry>
4147 <entry>Value</entry>
4148 <entry>Description</entry>
4153 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4157 If an application A specifies this flag and succeeds in
4158 becoming the owner of the name, and another application B
4159 later calls RequestName with the
4160 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4161 will lose ownership and receive a
4162 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4163 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4164 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4165 is not specified by application B, then application B will not replace
4166 application A as the owner.
4171 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4175 Try to replace the current owner if there is one. If this
4176 flag is not set the application will only become the owner of
4177 the name if there is no current owner. If this flag is set,
4178 the application will replace the current owner if
4179 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4184 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4188 Without this flag, if an application requests a name that is
4189 already owned, the application will be placed in a queue to
4190 own the name when the current owner gives it up. If this
4191 flag is given, the application will not be placed in the
4192 queue, the request for the name will simply fail. This flag
4193 also affects behavior when an application is replaced as
4194 name owner; by default the application moves back into the
4195 waiting queue, unless this flag was provided when the application
4196 became the name owner.
4204 The return code can be one of the following values:
4210 <entry>Conventional Name</entry>
4211 <entry>Value</entry>
4212 <entry>Description</entry>
4217 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4218 <entry>1</entry> <entry>The caller is now the primary owner of
4219 the name, replacing any previous owner. Either the name had no
4220 owner before, or the caller specified
4221 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4222 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4225 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4228 <entry>The name already had an owner,
4229 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4230 the current owner did not specify
4231 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4232 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4236 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4237 <entry>The name already has an owner,
4238 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4239 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4240 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4241 specified by the requesting application.</entry>
4244 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4246 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4254 <sect3 id="bus-messages-release-name">
4255 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4259 UINT32 ReleaseName (in STRING name)
4266 <entry>Argument</entry>
4268 <entry>Description</entry>
4274 <entry>STRING</entry>
4275 <entry>Name to release</entry>
4285 <entry>Argument</entry>
4287 <entry>Description</entry>
4293 <entry>UINT32</entry>
4294 <entry>Return value</entry>
4301 This method call should be sent to
4302 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4303 release the method caller's claim to the given name. If the caller is
4304 the primary owner, a new primary owner will be selected from the
4305 queue if any other owners are waiting. If the caller is waiting in
4306 the queue for the name, the caller will removed from the queue and
4307 will not be made an owner of the name if it later becomes available.
4308 If there are no other owners in the queue for the name, it will be
4309 removed from the bus entirely.
4311 The return code can be one of the following values:
4317 <entry>Conventional Name</entry>
4318 <entry>Value</entry>
4319 <entry>Description</entry>
4324 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4325 <entry>1</entry> <entry>The caller has released his claim on
4326 the given name. Either the caller was the primary owner of
4327 the name, and the name is now unused or taken by somebody
4328 waiting in the queue for the name, or the caller was waiting
4329 in the queue for the name and has now been removed from the
4333 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4335 <entry>The given name does not exist on this bus.</entry>
4338 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4340 <entry>The caller was not the primary owner of this name,
4341 and was also not waiting in the queue to own this name.</entry>
4349 <sect3 id="bus-messages-list-queued-owners">
4350 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4354 ARRAY of STRING ListQueuedOwners (in STRING name)
4361 <entry>Argument</entry>
4363 <entry>Description</entry>
4369 <entry>STRING</entry>
4370 <entry>The well-known bus name to query, such as
4371 <literal>com.example.cappuccino</literal></entry>
4381 <entry>Argument</entry>
4383 <entry>Description</entry>
4389 <entry>ARRAY of STRING</entry>
4390 <entry>The unique bus names of connections currently queued
4391 for the name</entry>
4398 This method call should be sent to
4399 <literal>org.freedesktop.DBus</literal> and lists the connections
4400 currently queued for a bus name (see
4401 <xref linkend="term-queued-owner"/>).
4406 <sect2 id="message-bus-routing">
4407 <title>Message Bus Message Routing</title>
4410 Messages may have a <literal>DESTINATION</literal> field (see <xref
4411 linkend="message-protocol-header-fields"/>), resulting in a
4412 <firstterm>unicast message</firstterm>. If the
4413 <literal>DESTINATION</literal> field is present, it specifies a message
4414 recipient by name. Method calls and replies normally specify this field.
4415 The message bus must send messages (of any type) with the
4416 <literal>DESTINATION</literal> field set to the specified recipient,
4417 regardless of whether the recipient has set up a match rule matching
4422 When the message bus receives a signal, if the
4423 <literal>DESTINATION</literal> field is absent, it is considered to
4424 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4425 applications with <firstterm>message matching rules</firstterm> that
4426 match the message. Most signal messages are broadcasts.
4430 Unicast signal messages (those with a <literal>DESTINATION</literal>
4431 field) are not commonly used, but they are treated like any unicast
4432 message: they are delivered to the specified receipient,
4433 regardless of its match rules. One use for unicast signals is to
4434 avoid a race condition in which a signal is emitted before the intended
4435 recipient can call <xref linkend="bus-messages-add-match"/> to
4436 receive that signal: if the signal is sent directly to that recipient
4437 using a unicast message, it does not need to add a match rule at all,
4438 and there is no race condition. Another use for unicast signals,
4439 on message buses whose security policy prevents eavesdropping, is to
4440 send sensitive information which should only be visible to one
4445 When the message bus receives a method call, if the
4446 <literal>DESTINATION</literal> field is absent, the call is taken to be
4447 a standard one-to-one message and interpreted by the message bus
4448 itself. For example, sending an
4449 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4450 <literal>DESTINATION</literal> will cause the message bus itself to
4451 reply to the ping immediately; the message bus will not make this
4452 message visible to other applications.
4456 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4457 the ping message were sent with a <literal>DESTINATION</literal> name of
4458 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4459 forwarded, and the Yoyodyne Corporation screensaver application would be
4460 expected to reply to the ping.
4464 Message bus implementations may impose a security policy which
4465 prevents certain messages from being sent or received.
4466 When a message cannot be sent or received due to a security
4467 policy, the message bus should send an error reply, unless the
4468 original message had the <literal>NO_REPLY</literal> flag.
4471 <sect3 id="message-bus-routing-eavesdropping">
4472 <title>Eavesdropping</title>
4474 Receiving a unicast message whose <literal>DESTINATION</literal>
4475 indicates a different recipient is called
4476 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4477 a security boundary (like the standard system bus), the security
4478 policy should usually prevent eavesdropping, since unicast messages
4479 are normally kept private and may contain security-sensitive
4484 Eavesdropping is mainly useful for debugging tools, such as
4485 the <literal>dbus-monitor</literal> tool in the reference
4486 implementation of D-Bus. Tools which eavesdrop on the message bus
4487 should be careful to avoid sending a reply or error in response to
4488 messages intended for a different client.
4492 Clients may attempt to eavesdrop by adding match rules
4493 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4494 the <literal>eavesdrop='true'</literal> match. If the message bus'
4495 security policy does not allow eavesdropping, the match rule can
4496 still be added, but will not have any practical effect. For
4497 compatibility with older message bus implementations, if adding such
4498 a match rule results in an error reply, the client may fall back to
4499 adding the same rule with the <literal>eavesdrop</literal> match
4504 <sect3 id="message-bus-routing-match-rules">
4505 <title>Match Rules</title>
4507 An important part of the message bus routing protocol is match
4508 rules. Match rules describe the messages that should be sent to a
4509 client, based on the contents of the message. Broadcast signals
4510 are only sent to clients which have a suitable match rule: this
4511 avoids waking up client processes to deal with signals that are
4512 not relevant to that client.
4515 Messages that list a client as their <literal>DESTINATION</literal>
4516 do not need to match the client's match rules, and are sent to that
4517 client regardless. As a result, match rules are mainly used to
4518 receive a subset of broadcast signals.
4521 Match rules can also be used for eavesdropping
4522 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4523 if the security policy of the message bus allows it.
4526 Match rules are added using the AddMatch bus method
4527 (see <xref linkend="bus-messages-add-match"/>). Rules are
4528 specified as a string of comma separated key/value pairs.
4529 Excluding a key from the rule indicates a wildcard match.
4530 For instance excluding the the member from a match rule but
4531 adding a sender would let all messages from that sender through.
4532 An example of a complete rule would be
4533 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4536 The following table describes the keys that can be used to create
4543 <entry>Possible Values</entry>
4544 <entry>Description</entry>
4549 <entry><literal>type</literal></entry>
4550 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4551 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4554 <entry><literal>sender</literal></entry>
4555 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4556 and <xref linkend="term-unique-name"/> respectively)
4558 <entry>Match messages sent by a particular sender. An example of a sender match
4559 is sender='org.freedesktop.Hal'</entry>
4562 <entry><literal>interface</literal></entry>
4563 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4564 <entry>Match messages sent over or to a particular interface. An example of an
4565 interface match is interface='org.freedesktop.Hal.Manager'.
4566 If a message omits the interface header, it must not match any rule
4567 that specifies this key.</entry>
4570 <entry><literal>member</literal></entry>
4571 <entry>Any valid method or signal name</entry>
4572 <entry>Matches messages which have the give method or signal name. An example of
4573 a member match is member='NameOwnerChanged'</entry>
4576 <entry><literal>path</literal></entry>
4577 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4578 <entry>Matches messages which are sent from or to the given object. An example of a
4579 path match is path='/org/freedesktop/Hal/Manager'</entry>
4582 <entry><literal>path_namespace</literal></entry>
4583 <entry>An object path</entry>
4586 Matches messages which are sent from or to an
4587 object for which the object path is either the
4588 given value, or that value followed by one or
4589 more path components.
4594 <literal>path_namespace='/com/example/foo'</literal>
4595 would match signals sent by
4596 <literal>/com/example/foo</literal>
4598 <literal>/com/example/foo/bar</literal>,
4600 <literal>/com/example/foobar</literal>.
4604 Using both <literal>path</literal> and
4605 <literal>path_namespace</literal> in the same match
4606 rule is not allowed.
4611 This match key was added in version 0.16 of the
4612 D-Bus specification and implemented by the bus
4613 daemon in dbus 1.5.0 and later.
4619 <entry><literal>destination</literal></entry>
4620 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4621 <entry>Matches messages which are being sent to the given unique name. An
4622 example of a destination match is destination=':1.0'</entry>
4625 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4626 <entry>Any string</entry>
4627 <entry>Arg matches are special and are used for further restricting the
4628 match based on the arguments in the body of a message. Only arguments of type
4629 STRING can be matched in this way. An example of an argument match
4630 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4634 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4635 <entry>Any string</entry>
4637 <para>Argument path matches provide a specialised form of wildcard matching for
4638 path-like namespaces. They can match arguments whose type is either STRING or
4639 OBJECT_PATH. As with normal argument matches,
4640 if the argument is exactly equal to the string given in the match
4641 rule then the rule is satisfied. Additionally, there is also a
4642 match when either the string given in the match rule or the
4643 appropriate message argument ends with '/' and is a prefix of the
4644 other. An example argument path match is arg0path='/aa/bb/'. This
4645 would match messages with first arguments of '/', '/aa/',
4646 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4647 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4649 <para>This is intended for monitoring “directories” in file system-like
4650 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4651 system. An application interested in all nodes in a particular hierarchy would
4652 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4653 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4654 represent a modification to the “bar” property, or a signal with zeroth
4655 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4656 many properties within that directory, and the interested application would be
4657 notified in both cases.</para>
4660 This match key was added in version 0.12 of the
4661 D-Bus specification, implemented for STRING
4662 arguments by the bus daemon in dbus 1.2.0 and later,
4663 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4670 <entry><literal>arg0namespace</literal></entry>
4671 <entry>Like a bus name, except that the string is not
4672 required to contain a '.' (period)</entry>
4674 <para>Match messages whose first argument is of type STRING, and is a bus name
4675 or interface name within the specified namespace. This is primarily intended
4676 for watching name owner changes for a group of related bus names, rather than
4677 for a single name or all name changes.</para>
4679 <para>Because every valid interface name is also a valid
4680 bus name, this can also be used for messages whose
4681 first argument is an interface name.</para>
4683 <para>For example, the match rule
4684 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4685 matches name owner changes for bus names such as
4686 <literal>com.example.backend.foo</literal>,
4687 <literal>com.example.backend.foo.bar</literal>, and
4688 <literal>com.example.backend</literal> itself.</para>
4690 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4693 This match key was added in version 0.16 of the
4694 D-Bus specification and implemented by the bus
4695 daemon in dbus 1.5.0 and later.
4701 <entry><literal>eavesdrop</literal></entry>
4702 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4703 <entry>Since D-Bus 1.5.6, match rules do not
4704 match messages which have a <literal>DESTINATION</literal>
4705 field unless the match rule specifically
4707 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4708 by specifying <literal>eavesdrop='true'</literal>
4709 in the match rule. <literal>eavesdrop='false'</literal>
4710 restores the default behaviour. Messages are
4711 delivered to their <literal>DESTINATION</literal>
4712 regardless of match rules, so this match does not
4713 affect normal delivery of unicast messages.
4714 If the message bus has a security policy which forbids
4715 eavesdropping, this match may still be used without error,
4716 but will not have any practical effect.
4717 In older versions of D-Bus, this match was not allowed
4718 in match rules, and all match rules behaved as if
4719 <literal>eavesdrop='true'</literal> had been used.
4728 <sect2 id="message-bus-starting-services">
4729 <title>Message Bus Starting Services</title>
4731 The message bus can start applications on behalf of other applications.
4732 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4733 An application that can be started in this way is called a
4734 <firstterm>service</firstterm>.
4737 With D-Bus, starting a service is normally done by name. That is,
4738 applications ask the message bus to start some program that will own a
4739 well-known name, such as <literal>com.example.TextEditor</literal>.
4740 This implies a contract documented along with the name
4741 <literal>com.example.TextEditor</literal> for which object
4742 the owner of that name will provide, and what interfaces those
4746 To find an executable corresponding to a particular name, the bus daemon
4747 looks for <firstterm>service description files</firstterm>. Service
4748 description files define a mapping from names to executables. Different
4749 kinds of message bus will look for these files in different places, see
4750 <xref linkend="message-bus-types"/>.
4753 Service description files have the ".service" file
4754 extension. The message bus will only load service description files
4755 ending with .service; all other files will be ignored. The file format
4756 is similar to that of <ulink
4757 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4758 entries</ulink>. All service description files must be in UTF-8
4759 encoding. To ensure that there will be no name collisions, service files
4760 must be namespaced using the same mechanism as messages and service
4765 On the well-known system bus, the name of a service description file
4766 must be its well-known name plus <literal>.service</literal>,
4768 <literal>com.example.ConfigurationDatabase.service</literal>.
4772 On the well-known session bus, services should follow the same
4773 service description file naming convention as on the system bus,
4774 but for backwards compatibility they are not required to do so.
4778 [FIXME the file format should be much better specified than "similar to
4779 .desktop entries" esp. since desktop entries are already
4780 badly-specified. ;-)]
4781 These sections from the specification apply to service files as well:
4784 <listitem><para>General syntax</para></listitem>
4785 <listitem><para>Comment format</para></listitem>
4788 Service description files must contain a
4789 <literal>D-BUS Service</literal> group with at least the keys
4790 <literal>Name</literal> (the well-known name of the service)
4791 and <literal>Exec</literal> (the command to be executed).
4794 <title>Example service description file</title>
4796 # Sample service description file
4798 Name=com.example.ConfigurationDatabase
4799 Exec=/usr/bin/sample-configd
4805 Additionally, service description files for the well-known system
4806 bus on Unix must contain a <literal>User</literal> key, whose value
4807 is the name of a user account (e.g. <literal>root</literal>).
4808 The system service will be run as that user.
4812 When an application asks to start a service by name, the bus daemon tries to
4813 find a service that will own that name. It then tries to spawn the
4814 executable associated with it. If this fails, it will report an
4819 On the well-known system bus, it is not possible for two .service files
4820 in the same directory to offer the same service, because they are
4821 constrained to have names that match the service name.
4825 On the well-known session bus, if two .service files in the same
4826 directory offer the same service name, the result is undefined.
4827 Distributors should avoid this situation, for instance by naming
4828 session services' .service files according to their service name.
4832 If two .service files in different directories offer the same
4833 service name, the one in the higher-priority directory is used:
4834 for instance, on the system bus, .service files in
4835 /usr/local/share/dbus-1/system-services take precedence over those
4836 in /usr/share/dbus-1/system-services.
4839 The executable launched will have the environment variable
4840 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4841 message bus so it can connect and request the appropriate names.
4844 The executable being launched may want to know whether the message bus
4845 starting it is one of the well-known message buses (see <xref
4846 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4847 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4848 of the well-known buses. The currently-defined values for this variable
4849 are <literal>system</literal> for the systemwide message bus,
4850 and <literal>session</literal> for the per-login-session message
4851 bus. The new executable must still connect to the address given
4852 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4853 resulting connection is to the well-known bus.
4856 [FIXME there should be a timeout somewhere, either specified
4857 in the .service file, by the client, or just a global value
4858 and if the client being activated fails to connect within that
4859 timeout, an error should be sent back.]
4862 <sect3 id="message-bus-starting-services-scope">
4863 <title>Message Bus Service Scope</title>
4865 The "scope" of a service is its "per-", such as per-session,
4866 per-machine, per-home-directory, or per-display. The reference
4867 implementation doesn't yet support starting services in a different
4868 scope from the message bus itself. So e.g. if you start a service
4869 on the session bus its scope is per-session.
4872 We could add an optional scope to a bus name. For example, for
4873 per-(display,session pair), we could have a unique ID for each display
4874 generated automatically at login and set on screen 0 by executing a
4875 special "set display ID" binary. The ID would be stored in a
4876 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4877 random bytes. This ID would then be used to scope names.
4878 Starting/locating a service could be done by ID-name pair rather than
4882 Contrast this with a per-display scope. To achieve that, we would
4883 want a single bus spanning all sessions using a given display.
4884 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4885 property on screen 0 of the display, pointing to this bus.
4890 <sect2 id="message-bus-types">
4891 <title>Well-known Message Bus Instances</title>
4893 Two standard message bus instances are defined here, along with how
4894 to locate them and where their service files live.
4896 <sect3 id="message-bus-types-login">
4897 <title>Login session message bus</title>
4899 Each time a user logs in, a <firstterm>login session message
4900 bus</firstterm> may be started. All applications in the user's login
4901 session may interact with one another using this message bus.
4904 The address of the login session message bus is given
4905 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4906 variable. If that variable is not set, applications may
4907 also try to read the address from the X Window System root
4908 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4909 The root window property must have type <literal>STRING</literal>.
4910 The environment variable should have precedence over the
4911 root window property.
4913 <para>The address of the login session message bus is given in the
4914 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4915 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4916 "autolaunch:", the system should use platform-specific methods of
4917 locating a running D-Bus session server, or starting one if a running
4918 instance cannot be found. Note that this mechanism is not recommended
4919 for attempting to determine if a daemon is running. It is inherently
4920 racy to attempt to make this determination, since the bus daemon may
4921 be started just before or just after the determination is made.
4922 Therefore, it is recommended that applications do not try to make this
4923 determination for their functionality purposes, and instead they
4924 should attempt to start the server.</para>
4926 <sect4 id="message-bus-types-login-x-windows">
4927 <title>X Windowing System</title>
4929 For the X Windowing System, the application must locate the
4930 window owner of the selection represented by the atom formed by
4934 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4938 <para>the current user's username</para>
4942 <para>the literal character '_' (underscore)</para>
4946 <para>the machine's ID</para>
4952 The following properties are defined for the window that owns
4954 <informaltable frame="all">
4963 <para>meaning</para>
4969 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4973 <para>the actual address of the server socket</para>
4979 <para>_DBUS_SESSION_BUS_PID</para>
4983 <para>the PID of the server process</para>
4992 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4993 present in this window.
4997 If the X selection cannot be located or if reading the
4998 properties from the window fails, the implementation MUST conclude
4999 that there is no D-Bus server running and proceed to start a new
5000 server. (See below on concurrency issues)
5004 Failure to connect to the D-Bus server address thus obtained
5005 MUST be treated as a fatal connection error and should be reported
5010 As an alternative, an implementation MAY find the information
5011 in the following file located in the current user's home directory,
5012 in subdirectory .dbus/session-bus/:
5015 <para>the machine's ID</para>
5019 <para>the literal character '-' (dash)</para>
5023 <para>the X display without the screen number, with the
5024 following prefixes removed, if present: ":", "localhost:"
5025 ."localhost.localdomain:". That is, a display of
5026 "localhost:10.0" produces just the number "10"</para>
5032 The contents of this file NAME=value assignment pairs and
5033 lines starting with # are comments (no comments are allowed
5034 otherwise). The following variable names are defined:
5041 <para>Variable</para>
5045 <para>meaning</para>
5051 <para>DBUS_SESSION_BUS_ADDRESS</para>
5055 <para>the actual address of the server socket</para>
5061 <para>DBUS_SESSION_BUS_PID</para>
5065 <para>the PID of the server process</para>
5071 <para>DBUS_SESSION_BUS_WINDOWID</para>
5075 <para>the window ID</para>
5084 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
5089 Failure to open this file MUST be interpreted as absence of a
5090 running server. Therefore, the implementation MUST proceed to
5091 attempting to launch a new bus server if the file cannot be
5096 However, success in opening this file MUST NOT lead to the
5097 conclusion that the server is running. Thus, a failure to connect to
5098 the bus address obtained by the alternative method MUST NOT be
5099 considered a fatal error. If the connection cannot be established,
5100 the implementation MUST proceed to check the X selection settings or
5101 to start the server on its own.
5105 If the implementation concludes that the D-Bus server is not
5106 running it MUST attempt to start a new server and it MUST also
5107 ensure that the daemon started as an effect of the "autolaunch"
5108 mechanism provides the lookup mechanisms described above, so
5109 subsequent calls can locate the newly started server. The
5110 implementation MUST also ensure that if two or more concurrent
5111 initiations happen, only one server remains running and all other
5112 initiations are able to obtain the address of this server and
5113 connect to it. In other words, the implementation MUST ensure that
5114 the X selection is not present when it attempts to set it, without
5115 allowing another process to set the selection between the
5116 verification and the setting (e.g., by using XGrabServer /
5123 On Unix systems, the session bus should search for .service files
5124 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5126 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5127 Implementations may also search additional locations, which
5128 should be searched with lower priority than anything in
5129 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5130 for example, the reference implementation also
5131 looks in <literal>${datadir}/dbus-1/services</literal> as
5132 set at compile time.
5135 As described in the XDG Base Directory Specification, software
5136 packages should install their session .service files to their
5137 configured <literal>${datadir}/dbus-1/services</literal>,
5138 where <literal>${datadir}</literal> is as defined by the GNU
5139 coding standards. System administrators or users can arrange
5140 for these service files to be read by setting XDG_DATA_DIRS or by
5141 symlinking them into the default locations.
5145 <sect3 id="message-bus-types-system">
5146 <title>System message bus</title>
5148 A computer may have a <firstterm>system message bus</firstterm>,
5149 accessible to all applications on the system. This message bus may be
5150 used to broadcast system events, such as adding new hardware devices,
5151 changes in the printer queue, and so forth.
5154 The address of the system message bus is given
5155 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5156 variable. If that variable is not set, applications should try
5157 to connect to the well-known address
5158 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5161 The D-Bus reference implementation actually honors the
5162 <literal>$(localstatedir)</literal> configure option
5163 for this address, on both client and server side.
5168 On Unix systems, the system bus should default to searching
5169 for .service files in
5170 <literal>/usr/local/share/dbus-1/system-services</literal>,
5171 <literal>/usr/share/dbus-1/system-services</literal> and
5172 <literal>/lib/dbus-1/system-services</literal>, with that order
5173 of precedence. It may also search other implementation-specific
5174 locations, but should not vary these locations based on environment
5178 The system bus is security-sensitive and is typically executed
5179 by an init system with a clean environment. Its launch helper
5180 process is particularly security-sensitive, and specifically
5181 clears its own environment.
5186 Software packages should install their system .service
5187 files to their configured
5188 <literal>${datadir}/dbus-1/system-services</literal>,
5189 where <literal>${datadir}</literal> is as defined by the GNU
5190 coding standards. System administrators can arrange
5191 for these service files to be read by editing the system bus'
5192 configuration file or by symlinking them into the default
5198 <sect2 id="message-bus-messages">
5199 <title>Message Bus Messages</title>
5201 The special message bus name <literal>org.freedesktop.DBus</literal>
5202 responds to a number of additional messages.
5205 <sect3 id="bus-messages-hello">
5206 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5217 <entry>Argument</entry>
5219 <entry>Description</entry>
5225 <entry>STRING</entry>
5226 <entry>Unique name assigned to the connection</entry>
5233 Before an application is able to send messages to other applications
5234 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5235 to the message bus to obtain a unique name. If an application without
5236 a unique name tries to send a message to another application, or a
5237 message to the message bus itself that isn't the
5238 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5239 disconnected from the bus.
5242 There is no corresponding "disconnect" request; if a client wishes to
5243 disconnect from the bus, it simply closes the socket (or other
5244 communication channel).
5247 <sect3 id="bus-messages-list-names">
5248 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5252 ARRAY of STRING ListNames ()
5259 <entry>Argument</entry>
5261 <entry>Description</entry>
5267 <entry>ARRAY of STRING</entry>
5268 <entry>Array of strings where each string is a bus name</entry>
5275 Returns a list of all currently-owned names on the bus.
5278 <sect3 id="bus-messages-list-activatable-names">
5279 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5283 ARRAY of STRING ListActivatableNames ()
5290 <entry>Argument</entry>
5292 <entry>Description</entry>
5298 <entry>ARRAY of STRING</entry>
5299 <entry>Array of strings where each string is a bus name</entry>
5306 Returns a list of all names that can be activated on the bus.
5309 <sect3 id="bus-messages-name-exists">
5310 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5314 BOOLEAN NameHasOwner (in STRING name)
5321 <entry>Argument</entry>
5323 <entry>Description</entry>
5329 <entry>STRING</entry>
5330 <entry>Name to check</entry>
5340 <entry>Argument</entry>
5342 <entry>Description</entry>
5348 <entry>BOOLEAN</entry>
5349 <entry>Return value, true if the name exists</entry>
5356 Checks if the specified name exists (currently has an owner).
5360 <sect3 id="bus-messages-name-owner-changed">
5361 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5365 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5372 <entry>Argument</entry>
5374 <entry>Description</entry>
5380 <entry>STRING</entry>
5381 <entry>Name with a new owner</entry>
5385 <entry>STRING</entry>
5386 <entry>Old owner or empty string if none</entry>
5390 <entry>STRING</entry>
5391 <entry>New owner or empty string if none</entry>
5398 This signal indicates that the owner of a name has changed.
5399 It's also the signal to use to detect the appearance of
5400 new names on the bus.
5403 <sect3 id="bus-messages-name-lost">
5404 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5408 NameLost (STRING name)
5415 <entry>Argument</entry>
5417 <entry>Description</entry>
5423 <entry>STRING</entry>
5424 <entry>Name which was lost</entry>
5431 This signal is sent to a specific application when it loses
5432 ownership of a name.
5436 <sect3 id="bus-messages-name-acquired">
5437 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5441 NameAcquired (STRING name)
5448 <entry>Argument</entry>
5450 <entry>Description</entry>
5456 <entry>STRING</entry>
5457 <entry>Name which was acquired</entry>
5464 This signal is sent to a specific application when it gains
5465 ownership of a name.
5469 <sect3 id="bus-messages-start-service-by-name">
5470 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5474 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5481 <entry>Argument</entry>
5483 <entry>Description</entry>
5489 <entry>STRING</entry>
5490 <entry>Name of the service to start</entry>
5494 <entry>UINT32</entry>
5495 <entry>Flags (currently not used)</entry>
5505 <entry>Argument</entry>
5507 <entry>Description</entry>
5513 <entry>UINT32</entry>
5514 <entry>Return value</entry>
5519 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5523 The return value can be one of the following values:
5528 <entry>Identifier</entry>
5529 <entry>Value</entry>
5530 <entry>Description</entry>
5535 <entry>DBUS_START_REPLY_SUCCESS</entry>
5537 <entry>The service was successfully started.</entry>
5540 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5542 <entry>A connection already owns the given name.</entry>
5551 <sect3 id="bus-messages-update-activation-environment">
5552 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5556 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5563 <entry>Argument</entry>
5565 <entry>Description</entry>
5571 <entry>ARRAY of DICT<STRING,STRING></entry>
5572 <entry>Environment to add or update</entry>
5577 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5580 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5583 Note, both the environment variable names and values must be valid UTF-8. There's no way to update the activation environment with data that is invalid UTF-8.
5588 <sect3 id="bus-messages-get-name-owner">
5589 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5593 STRING GetNameOwner (in STRING name)
5600 <entry>Argument</entry>
5602 <entry>Description</entry>
5608 <entry>STRING</entry>
5609 <entry>Name to get the owner of</entry>
5619 <entry>Argument</entry>
5621 <entry>Description</entry>
5627 <entry>STRING</entry>
5628 <entry>Return value, a unique connection name</entry>
5633 Returns the unique connection name of the primary owner of the name
5634 given. If the requested name doesn't have an owner, returns a
5635 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5639 <sect3 id="bus-messages-get-connection-unix-user">
5640 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5644 UINT32 GetConnectionUnixUser (in STRING bus_name)
5651 <entry>Argument</entry>
5653 <entry>Description</entry>
5659 <entry>STRING</entry>
5660 <entry>Unique or well-known bus name of the connection to
5661 query, such as <literal>:12.34</literal> or
5662 <literal>com.example.tea</literal></entry>
5672 <entry>Argument</entry>
5674 <entry>Description</entry>
5680 <entry>UINT32</entry>
5681 <entry>Unix user ID</entry>
5686 Returns the Unix user ID of the process connected to the server. If
5687 unable to determine it (for instance, because the process is not on the
5688 same machine as the bus daemon), an error is returned.
5692 <sect3 id="bus-messages-get-connection-unix-process-id">
5693 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5697 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5704 <entry>Argument</entry>
5706 <entry>Description</entry>
5712 <entry>STRING</entry>
5713 <entry>Unique or well-known bus name of the connection to
5714 query, such as <literal>:12.34</literal> or
5715 <literal>com.example.tea</literal></entry>
5725 <entry>Argument</entry>
5727 <entry>Description</entry>
5733 <entry>UINT32</entry>
5734 <entry>Unix process id</entry>
5739 Returns the Unix process ID of the process connected to the server. If
5740 unable to determine it (for instance, because the process is not on the
5741 same machine as the bus daemon), an error is returned.
5745 <sect3 id="bus-messages-get-connection-credentials">
5746 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
5750 DICT<STRING,VARIANT> GetConnectionCredentials (in STRING bus_name)
5757 <entry>Argument</entry>
5759 <entry>Description</entry>
5765 <entry>STRING</entry>
5766 <entry>Unique or well-known bus name of the connection to
5767 query, such as <literal>:12.34</literal> or
5768 <literal>com.example.tea</literal></entry>
5778 <entry>Argument</entry>
5780 <entry>Description</entry>
5786 <entry>DICT<STRING,VARIANT></entry>
5787 <entry>Credentials</entry>
5795 Returns as many credentials as possible for the process connected to
5796 the server. If unable to determine certain credentials (for instance,
5797 because the process is not on the same machine as the bus daemon,
5798 or because this version of the bus daemon does not support a
5799 particular security framework), or if the values of those credentials
5800 cannot be represented as documented here, then those credentials
5805 Keys in the returned dictionary not containing "." are defined
5806 by this specification. Bus daemon implementors supporting
5807 credentials frameworks not mentioned in this document should either
5808 contribute patches to this specification, or use keys containing
5809 "." and starting with a reversed domain name.
5815 <entry>Value type</entry>
5816 <entry>Value</entry>
5821 <entry>UnixUserID</entry>
5822 <entry>UINT32</entry>
5823 <entry>The numeric Unix user ID, as defined by POSIX</entry>
5826 <entry>ProcessID</entry>
5827 <entry>UINT32</entry>
5828 <entry>The numeric process ID, on platforms that have
5829 this concept. On Unix, this is the process ID defined by
5838 This method was added in D-Bus 1.7 to reduce the round-trips
5839 required to list a process's credentials. In older versions, calling
5840 this method will fail: applications should recover by using the
5841 separate methods such as
5842 <xref linkend="bus-messages-get-connection-unix-user"/>
5847 <sect3 id="bus-messages-get-adt-audit-session-data">
5848 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
5852 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
5859 <entry>Argument</entry>
5861 <entry>Description</entry>
5867 <entry>STRING</entry>
5868 <entry>Unique or well-known bus name of the connection to
5869 query, such as <literal>:12.34</literal> or
5870 <literal>com.example.tea</literal></entry>
5880 <entry>Argument</entry>
5882 <entry>Description</entry>
5888 <entry>ARRAY of BYTE</entry>
5889 <entry>auditing data as returned by
5890 adt_export_session_data()</entry>
5895 Returns auditing data used by Solaris ADT, in an unspecified
5896 binary format. If you know what this means, please contribute
5897 documentation via the D-Bus bug tracking system.
5898 This method is on the core DBus interface for historical reasons;
5899 the same information should be made available via
5900 <xref linkend="bus-messages-get-connection-credentials"/>
5905 <sect3 id="bus-messages-get-connection-selinux-security-context">
5906 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
5910 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
5917 <entry>Argument</entry>
5919 <entry>Description</entry>
5925 <entry>STRING</entry>
5926 <entry>Unique or well-known bus name of the connection to
5927 query, such as <literal>:12.34</literal> or
5928 <literal>com.example.tea</literal></entry>
5938 <entry>Argument</entry>
5940 <entry>Description</entry>
5946 <entry>ARRAY of BYTE</entry>
5947 <entry>some sort of string of bytes, not necessarily UTF-8,
5948 not including '\0'</entry>
5953 Returns the security context used by SELinux, in an unspecified
5954 format. If you know what this means, please contribute
5955 documentation via the D-Bus bug tracking system.
5956 This method is on the core DBus interface for historical reasons;
5957 the same information should be made available via
5958 <xref linkend="bus-messages-get-connection-credentials"/>
5964 <sect3 id="bus-messages-add-match">
5965 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5969 AddMatch (in STRING rule)
5976 <entry>Argument</entry>
5978 <entry>Description</entry>
5984 <entry>STRING</entry>
5985 <entry>Match rule to add to the connection</entry>
5990 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5991 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5995 <sect3 id="bus-messages-remove-match">
5996 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
6000 RemoveMatch (in STRING rule)
6007 <entry>Argument</entry>
6009 <entry>Description</entry>
6015 <entry>STRING</entry>
6016 <entry>Match rule to remove from the connection</entry>
6021 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
6022 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
6027 <sect3 id="bus-messages-get-id">
6028 <title><literal>org.freedesktop.DBus.GetId</literal></title>
6032 GetId (out STRING id)
6039 <entry>Argument</entry>
6041 <entry>Description</entry>
6047 <entry>STRING</entry>
6048 <entry>Unique ID identifying the bus daemon</entry>
6053 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
6054 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
6055 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
6056 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
6057 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
6058 by org.freedesktop.DBus.Peer.GetMachineId().
6059 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
6067 <appendix id="implementation-notes">
6068 <title>Implementation notes</title>
6069 <sect1 id="implementation-notes-subsection">
6077 <glossary><title>Glossary</title>
6079 This glossary defines some of the terms used in this specification.
6082 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
6085 The message bus maintains an association between names and
6086 connections. (Normally, there's one connection per application.) A
6087 bus name is simply an identifier used to locate connections. For
6088 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
6089 name might be used to send a message to a screensaver from Yoyodyne
6090 Corporation. An application is said to <firstterm>own</firstterm> a
6091 name if the message bus has associated the application's connection
6092 with the name. Names may also have <firstterm>queued
6093 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
6094 The bus assigns a unique name to each connection,
6095 see <xref linkend="term-unique-name"/>. Other names
6096 can be thought of as "well-known names" and are
6097 used to find applications that offer specific functionality.
6101 See <xref linkend="message-protocol-names-bus"/> for details of
6102 the syntax and naming conventions for bus names.
6107 <glossentry id="term-message"><glossterm>Message</glossterm>
6110 A message is the atomic unit of communication via the D-Bus
6111 protocol. It consists of a <firstterm>header</firstterm> and a
6112 <firstterm>body</firstterm>; the body is made up of
6113 <firstterm>arguments</firstterm>.
6118 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6121 The message bus is a special application that forwards
6122 or routes messages between a group of applications
6123 connected to the message bus. It also manages
6124 <firstterm>names</firstterm> used for routing
6130 <glossentry id="term-name"><glossterm>Name</glossterm>
6133 See <xref linkend="term-bus-name"/>. "Name" may
6134 also be used to refer to some of the other names
6135 in D-Bus, such as interface names.
6140 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6143 Used to prevent collisions when defining new interfaces, bus names
6144 etc. The convention used is the same one Java uses for defining
6145 classes: a reversed domain name.
6146 See <xref linkend="message-protocol-names-bus"/>,
6147 <xref linkend="message-protocol-names-interface"/>,
6148 <xref linkend="message-protocol-names-error"/>,
6149 <xref linkend="message-protocol-marshaling-object-path"/>.
6154 <glossentry id="term-object"><glossterm>Object</glossterm>
6157 Each application contains <firstterm>objects</firstterm>, which have
6158 <firstterm>interfaces</firstterm> and
6159 <firstterm>methods</firstterm>. Objects are referred to by a name,
6160 called a <firstterm>path</firstterm>.
6165 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6168 An application talking directly to another application, without going
6169 through a message bus. One-to-one connections may be "peer to peer" or
6170 "client to server." The D-Bus protocol has no concept of client
6171 vs. server after a connection has authenticated; the flow of messages
6172 is symmetrical (full duplex).
6177 <glossentry id="term-path"><glossterm>Path</glossterm>
6180 Object references (object names) in D-Bus are organized into a
6181 filesystem-style hierarchy, so each object is named by a path. As in
6182 LDAP, there's no difference between "files" and "directories"; a path
6183 can refer to an object, while still having child objects below it.
6188 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6191 Each bus name has a primary owner; messages sent to the name go to the
6192 primary owner. However, certain names also maintain a queue of
6193 secondary owners "waiting in the wings." If the primary owner releases
6194 the name, then the first secondary owner in the queue automatically
6195 becomes the new owner of the name.
6200 <glossentry id="term-service"><glossterm>Service</glossterm>
6203 A service is an executable that can be launched by the bus daemon.
6204 Services normally guarantee some particular features, for example they
6205 may guarantee that they will request a specific name such as
6206 "com.example.Screensaver", have a singleton object
6207 "/com/example/Application", and that object will implement the
6208 interface "com.example.Screensaver.Control".
6213 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6216 ".service files" tell the bus about service applications that can be
6217 launched (see <xref linkend="term-service"/>). Most importantly they
6218 provide a mapping from bus names to services that will request those
6219 names when they start up.
6224 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6227 The special name automatically assigned to each connection by the
6228 message bus. This name will never change owner, and will be unique
6229 (never reused during the lifetime of the message bus).
6230 It will begin with a ':' character.
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